The alerting effects of caffeine, bright light and face washing after a short daytime nap

Clinical Neurophysiology 114 (2003) 2268–2278 www.elsevier.com/locate/clinph

The alerting effects of caffeine, bright light and face washing after a short daytime nap Mitsuo Hayashi*, Akiko Masuda, Tadao Hori Department of Behavioral Sciences, Faculty of Integrated Arts and Sciences, Hiroshima University, 1-7-1, Kagamiyama, Higashi-Hiroshima 739-8521, Japan Accepted 14 July 2003

Abstract Objective: The present study examined whether the combination of a short daytime nap with caffeine, bright light and face washing was effective against mid-afternoon sleepiness. Methods: Ten young healthy adults participated in 5 experimental conditions; those experiments were—Nap only: taking a 20 min nap; Caffeine þ Nap: taking 200 mg of caffeine followed by a nap; Nap þ Bright-light: being exposed to 2000 lx of bright light for 1 min immediately after napping; Nap þ Face-washing: washing their faces immediately after napping; and No-Nap: taking a rest without sleep. These naps were taken at 12:40 hours. The subjects engaged in computer tasks for 15 min before napping and for 1 h after napping. Results: Caffeine þ Nap was the most effective for subjective sleepiness and performance level; its effects lasted throughout 1 h after napping. Nap þ Bright-light was comparable with Caffeine þ Nap, except for performance level. Nap þ Face-washing showed mild and transient effects, however, it suppressed subjective sleepiness immediately after napping. Conclusions: The effects of a short nap against mid-afternoon sleepiness could be enhanced by combining caffeine intake, exposure to bright light, or face washing. Significance: The present study would provide effective countermeasures against mid-afternoon sleepiness and sleepiness related accidents. q 2003 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved. Keywords: Nap; Sleepiness; Sleep inertia; Post-lunch dip; Caffeine; Bright-light

1. Introduction It is well known that sleepiness occurs and performance declines in the mid-afternoon. This so-called ‘post-lunch dip’ often causes sleepiness related accidents during that period (Mitler et al., 1988; Garbarino et al., 2001). This phenomenon occurs regardless of taking a lunchtime meal (Carskadon and Dement, 1992); it might reflect the circasemidians sleepiness cycle (Broughton, 1998; Hayashi et al., 2002). One of the countermeasures for sleepiness is napping. Recently, it was reported that short daytime naps of less than 30 min had positive effects on daytime alertness. These were experimentally confirmed after normal night sleep in young adults (Hayashi et al., 1999a,b, 2003) and elderly individuals (Tamaki et al., 1999, 2000; Tanaka et al., * Corresponding author. Tel.: þ81-824-24-6582; fax: þ 81-824-24-0759. E-mail address: [email protected] (M. Hayashi).

2001, 2002), after restricted night sleep (Gillberg et al., 1996; Horne and Reyner, 1996; Reyner and Horne, 1997; Stampi et al., 1990; Takahashi and Arito, 2000; Tietzel and Lack, 2001), during prolonged sustained performance (Naitoh et al., 1992) and night shift (Purnell et al., 2002). In the napping strategy at the work place, ‘sleep inertia’, which is enhancement of sleepiness or temporary decline in the performance level immediately after awakening, is one of the limiting factors (Muzet et al., 1995). Sleep inertia is enhanced when awakening directly from the deeper sleep stages (Ferrara and De Gennarro, 2000; Tassi and Muzet, 2000). A short daytime nap of less than 20 min consists of lighter sleep stages such as stages 1 or 2 sleep and rarely contains slow wave sleep (Stampi et al., 1990; Hayashi et al., 1999a,b). Therefore, in the short nap, the persons are awakened from lighter sleep stages so that sleep inertia is suppressed (Stampi et al., 1990; Tietzel and Lack, 2001). However, sleep inertia occurs briefly even after such short

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M. Hayashi et al. / Clinical Neurophysiology 114 (2003) 2268–2278

naps (Hayashi et al., 2003). Some countermeasures of sleep inertia are considered, such as physical or mental exercises, external noise, bright light, face washing using cold water, or psychostimulants (Ferrara and De Gennarro, 2000). However, the effects of these factors on sleep inertia have received little attention (Tassi and Muzet, 2000). Only few studies examined the effects of countermeasures such as acoustic noise (Tassi et al., 1992) or caffeine (Van Dongen et al., 2001). The combination of a short nap and countermeasures on sleep inertia would act more effectively on the post-lunch dip or sleepiness related accidents. Caffeine beverages are easily taken to maintain alertness. It enhances alertness and improves the performance level in various tasks (Van del Stelt and Snel, 1998), particularly under lowered alertness or enhanced fatigue (Bonnet and Arand, 1994; Bonnet et al., 1995; De Valck and Cluydts, 2001; Horne and Reyner, 1996; Lorist et al., 1994; Reyner and Horne, 1997, 2000; Smith, 1998). It is also an effective countermeasure against the post-lunch dip (Smith, 1998) or sleep inertia (Brice and Smith, 2002; Van Dongen et al., 2001). Plasma caffeine level peaks within 15– 120 min after oral ingestion, and 99% of caffeine is absorbed from the gastrointestinal tract within approximately 45 min (Arnaud, 1998). Therefore, ingestion of caffeine followed by a short nap would be effective to reduce sleep inertia. Reyner and Horne (1997) reported that the combination of caffeine and a short nap was more effective to prevent sleepiness after a restricted prior nocturnal sleep, in comparison with caffeine intake alone or taking a short nap alone. Bright light (. 2000 lx) suppresses the secretion of melatonin and prevents the decline of body temperature during the night, so that it enhances alertness during the night shift (Badia et al., 1991; Campbell and Dawson, 1990; Daurat et al., 1993; Dawson and Campbell, 1991; Myers and Badia, 1993; Cajochen et al., 2000). During the daytime, when melatonin was scarcely secreted, the effects of bright light were hardly confirmed in some studies, in which nocturnal sleep was deprived or restricted (Badia et al., 1991; Daurat et al., 1993; Lafrance et al., 1998; Leproult et al., 2001). Other studies, in which prior nocturnal sleep was not restricted, however, reported positive effects of bright light during the daytime in psychological and psychophysiological measures (Gru¨nberger et al., 1993; Saito et al., 1996). Therefore, if nocturnal sleep was not restricted, sudden bright light immediately after a short daytime nap may possibly act as an alerting stimulus to reduce sleep inertia. In daily life, people often wash their faces to fully awaken from sleep. A few studies confirmed the alerting effects of face washing with cold water immediately after waking (reviewed by Ferrara and De Gennarro, 2000). It was also reported that cold air had marginal and transient effects for car drivers (Reyner and Horne, 1998). Therefore, it can be expected that cold stimuli to the face by water would act positively on sleep inertia after a short nap.

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The aim of the present study was to examine the effects of caffeine, bright light and face washing on daytime sleepiness after daytime short naps. These conditions were compared with those of merely taking a nap and of not taking a nap. In the present study, sleepiness was evaluated by subjective, behavioral and physiological measures (Curcio et al., 2001; Jewett et al., 1999). Subjective ratings of sleepiness and fatigue were measured using a 100 mm long visual analog scale. This is sensitive to sleep deprivation and to time of day (Babkoff et al., 1991). While becoming sleepy, response omissions increase and reaction time is prolonged (Doran et al., 2001). As a behavioral parameter, these performance decrease measures were used. As physiological measures, P3 amplitude of event related potentials (ERP) and EEG power spectra were used. P3 amplitude reflects attention allocation and memory updating, while its amplitude attenuates as a function of the reduction of alertness (Polich and Kok, 1995). Insertion of theta and alpha activities in the EEG recordings with eyes ˚ kerstedt, 1991). opened reflected a lowered alertness (A Horne and Reyner (1996) and Reyner and Horne (1997, 1998, 2000) used 4 –11 Hz EEG activities as physiological sleepiness measures and confirmed the high validity. Young adults participated in the present study. There may be several limitations in interpretation of their data. One reason is that individuals at this age have fewer complaints about sleep problems. However, afternoon sleepiness does occur similar to other older age groups, even if they had a longer nocturnal sleep than their usual sleep time (Carskadon, 1989). Therefore, their data could be applied for other age groups. Another problem is that sleep habits of this age group are prone to be irregular and to be shortened. Therefore, subjects were recruited who had regular sleep – wake habits, and had normal nocturnal sleep lengths.

2. Methods 2.1. Subjects Ten university students (8 females and two males, 20– 23 years, mean 21.1 years) with good health participated in the study. They previously answered the sleep –wake habit inventory (Miyasita, 1994) and Morning – Evening ques¨ stberg, 1976). They reported that tionnaire (Horne and O they slept 6 – 8 h nightly, had normal sleep – wake habits, and did not complain of sleep – wake problems. They took naps less than once per week. They were good, non-irregular sleepers with normal sleep lengths. They were not excessive morning types nor evening types. All were nonsmokers, and consumed moderate amounts of caffeinated beverages (less than 400 mg of caffeine per day). They had also participated in another sleep study, and were confirmed to have no sleep – wake disorders. They gave informed consent prior to participation.

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2.2. Test sessions Each test session lasted for 5 min and was composed of subjective ratings of sleepiness and fatigue, and 4 min memory search task (Fig. 1). The subjects were engaged in 3 sessions (15 min) before and for 12 sessions (60 min) after the short nap. 2.2.1. Subjective ratings of sleepiness and fatigue The subjects rated their subjective sleepiness and fatigue using a 100 mm long visual analog scale (Monk, 1989) at the start of all the 5 min task sessions. The values of sleepiness and fatigue ranged from 0 (very alert or very vigorous) to 100 (very sleepy or very fatigued). 2.2.2. Task After rated subjective sleepiness and fatigue, the subjects engaged in 4 min memory search tasks. In this task, two of 20 alphabets, in which morphological resemblances were excluded (i.e., F, J, L, Q, R, V), were set as target stimuli and were displayed for 1.0 s at the start of the task. These target alphabets were randomly exchanged every session. Four seconds later, an alphabet sequence was randomly displayed for 200 ms at intervals of 1.0– 1.4 s (mean 1.2 s). The visual angle size of the stimulus was 0.868 wide and 1.248 high. Subjects were seated 60 cm from the computer screen and instructed to press a button with their right hand as quickly and accurately as possible when the target stimuli appeared. Two hundred trials were presented per session. Forty target stimuli were presented in a random order. 2.3. Procedure Before starting the experiment, the subjects sufficiently practiced the performance tasks. Excessive intake of alcohol and caffeine was prohibited during the experimental period. All subjects participated in 5 experimental conditions with intervals of more than 1 day. The experimental conditions were No-Nap, Nap, Face-washing, Bright-light and Caffeine. The experimental design is illustrated in Table 1. Three independent variables were examined: sleep (Nap or No-Nap), drug (Caffeine or placebo), and action (no-action, Face-washing or exposure to bright light) immediately after napping. These variables could be examined by contrasting with the following conditions; Sleep: the Nap versus No-Nap conditions; Drug: the Nap versus

Fig. 1. Schedule on the 5 min task session. VAS: self ratings of sleepiness and fatigue via visual analog scale.

Table 1 Experimental design Condition

No-Nap Nap Face-washing Bright-light Caffeine

Independent variables Sleep

Drug

Action

No-Nap Nap Nap Nap Nap

Placebo Placebo Placebo Placebo Caffeine

– – Face washing Exposure to bright light –

the Caffeine conditions; and Action: the Nap versus the Facewashing or the Bright-light conditions. It took approximately 2 weeks to complete all conditions per subject. To eliminate the order effects of the conditions, the order of the conditions was randomized across the subjects. The analysis of variances (ANOVAs) with repeated measures revealed that no order effects of conditions were observed in the sleep variables of a daytime nap and prior nocturnal sleep, and sleepiness evaluated by subjective, behavioral, and physiological measures. Therefore, any habituation to the experimental situations due to repeat testing over 5 days would be negligible. Between the experimental days, the subjects engaged in their normal daily activities. They took a nocturnal sleep at home and stayed in the laboratory from 11:00 to 14:30 hours during the experimental days. On the experimental days, caffeine intake was prohibited between arising in their home and reporting to the laboratory. They were instructed to follow their normal sleep – wake schedule. Sleep– wake habits were monitored by a sleep log and wrist activities (Mini-motion logger actigraph, Ambulatory Monitoring Inc., USA). They took a normal nocturnal sleep the day before carrying out each experimental condition. Mean nocturnal sleep among 5 nights prior to these experimental conditions was 437.3 min (S:D: ¼ 35:2) and was not significantly different among the conditions. The schedule of each experimental condition is illustrated in Fig. 2. They reported to the laboratory at 11:00 hours. Electrodes were attached to monitor EEG (Fz, Cz, Pz, C3 and O1), horizontal and vertical EOG and submentalis EMG activities. After taking lunch, they entered a soundproof and air-conditioned isolation unit and engaged in 5 min computer tasks for 3 sessions (15 min) from 12:15 hours. Five minutes before napping (12:35 hours), they drank 100 ml of coffee. In the Caffeine condition, this was 2 g of decaffeinated instant coffee mixed with 200 mg of anhydrous caffeine and 5 ml of fresh cream. No subjects took sugar. In the other 4 conditions, they drank an equal amount of decaffeinated coffee with 5 ml of fresh cream as placebo. The fresh cream was mixed to prevent discriminating the bitter taste of caffeinated from decaffeinated coffee. Retrospective reports by the subjects after terminating all experimental conditions showed that they did not notice the difference between them.

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In the Nap, Caffeine, and Bright-light conditions, they sat in a chair within 1 min after waking from the nap. In the Bright-light condition, they were exposed to bright light for 1 min sitting in the chair. A mobile panel of mounted fluorescent tubes was placed in front of the subjects with an approximate distance of 70 cm from the center of the panel to the subjects’ faces. The illumination was adjusted to approximately 2000 lx at their eye level. Two minutes after waking from the nap in the Nap, Caffeine, Face-washing, and Bright-light conditions, or 2 min after resting in the No-Nap condition, the 5 min computer sessions started again and lasted for 60 min (5 min £ 12 sessions). 2.4. Recording and data analysis

Fig. 2. Schedule on the experimental day. (a) In the Caffeine condition, the subjects drank 100 ml of coffee containing 200 mg of caffeine, while they drank a similar volume of decaffeinated coffee in the other 4 conditions. (b) In the No-Nap condition, the subjects read a newspaper (Rest), while they took a nap for 15 min in the other 4 conditions. (c) In the Bright-light condition, the subjects were exposed to 2000 lx of illumination for 1 min. In the Face-washing condition, they washed their face sitting on the edge of the bed, and sat in the chair.

They read a newspaper sitting on a semi-reclining chair from 12:40 to 13:00 hours in the No-Nap condition. The EEG recordings confirmed that they remained awake throughout that time. In the other 4 conditions, they lay in bed at 12:40 hours and were awakened using an intercom when 15 min had elapsed from the onset of sleep stage 1 (Rechtschaffen and Kales, 1968). If they spontaneously awoke from any sleep stage before 15 min had elapsed, they remained in bed until a total of 15 min of any sleep stage had accumulated. Immediately upon waking from the nap, they answered verbally their estimated nap time (min), sleep time (min), and satisfaction regarding the nap and their mood using a 5-point scale (5: very good, 1: very poor). In the Face-washing condition, a washbowl containing 2 l of water at 25 ^ 2 8C was set on a stand at a height of 88 cm in front of the bed. The subjects washed their faces sitting on the edge of the bed within 2 min after waking from the nap. Therefore, their physical activities accompanied by the face washing were somewhat restricted. After face washing, they sat in a chair and waited to start the post-nap sessions.

2.4.1. Polygraph recordings EEG, EOG and EMG activities were recorded using a 14channel electroencephalograph (NEC San-ei 1A97) and FM tape recorder (TEAC SR-50). EEG recordings were referred to linked mastoids. The vertical and horizontal EOGs were recorded from above and to the right of the right eye. Interelectrode impedance was below 5 kV. The EEG of the Fz, Cz and Pz areas was amplified using a time constant of 3.2 s, because of the measurement for ERP, while the C3 and O1 areas of 0.3 s because of scoring for sleep stages. The EOG and EMG were amplified by 1.5 and 0.03 s, respectively. 2.4.2. Sleep stages during the nap Sleep stages during the nap were scored in 30 s epochs using standard criteria (Rechtschaffen and Kales, 1968). 2.4.3. Behavioral measures During the memory search task, the time elapsed between the onset of target stimuli and button presses by the subjects was measured as reaction time. Reaction times in the correct responses were averaged per session. In addition, the percentage of the trials in which the subjects missed the target was calculated as the percent miss. 2.4.4. Event related potentials To measure the ERP to the target stimuli during the memory search task, artifact-free EEG data were sampled at 200 Hz with band-pass filters of 0.05 –60 Hz from 100 before to 900 ms after the stimuli were presented. The recordings contaminated by vertical and horizontal eye movements greater than 80 mV were excluded from analysis. The averaged amplitudes from 100 ms before until the onset of the stimuli were subtracted from the original ERP waveform. The waveforms of correct responses were averaged per session using an NEC San-ei Signal-Processor (model 7T18A). The maximum amplitude between 300 and 600 ms after the onset of stimuli was identified as the P3 amplitude. This P3 component was considered to be the P3b (Friedman et al., 1997), so that

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the P3 amplitudes at the Pz area were used in the analysis. Since eye-blinking artifacts frequently contaminated the No-Nap condition for 5 of 10 subjects, less than 20 ERP samples to the target stimuli were available in this condition. Therefore, ERP data in the No-Nap condition were excluded from the analysis. 2.4.5. EEG activities Artifact-free EEG data of 5.12 s lengths at the C3 area were sampled at 200 Hz with band-pass filters of 0.5 – 60 Hz and were spectrally analyzed with 0.195 Hz resolutions using a fast Fourier transformation with an NEC San-ei Signal-Processor (model 7T18A). The recordings contaminated by vertical and horizontal eye movements greater than 80 mV were excluded from the analysis. The EEG spectra per session were obtained by averaging more than six 5.12 s epochs. The power spectra were integrated for 4.0– 11.0 Hz band frequencies, and transformed into the magnitude values in microvolts. 2.4.6. Statistical analysis One-way (4 conditions except for the No-Nap condition) ANOVAs with repeated measures were performed for the sleep variables and subjective ratings of the nap. The degrees of freedom were adjusted by Huynh and Feldt’s epsilon (Winer et al., 1991). The post hoc comparisons were performed using the Newman – Keuls procedure. The significance level was set at 0.05. The values in subjective ratings of sleepiness and fatigue, reaction time and percent miss in the memory search task, the P3 amplitude at the Pz area and EEG 4 –11 Hz band activity at the C3 area were averaged every 3 sessions, and

then two-way [5ðconditionsÞ £ 5ðblocks of 3 sessionsÞ] ANOVAs with repeated measures were performed. In addition, to examine the effect of sleep inertia immediately after napping, two-way [5ðconditionsÞ£ 3ðsessions immediately after nappingÞ] ANOVAs with repeated measures were also performed. The data of 3 sessions immediately after napping were previously transformed as the mean values of pre-nap sessions set to the 0 level.

3. Results 3.1. Sleep variables and subjective ratings of the nap Table 2 shows the sleep variables and subjective ratings of the naps. The subjects actually slept for 13 – 16 min except for two subjects in the Caffeine condition. Although these two subjects spent time in bed for 30 min, they could sleep for only 1.5 and 7 min, respectively. Their data were also included in the statistical analysis because their results obtained in the subjective, behavioral and physiological sleepiness measures were similar to the other 8 subjects. All naps were composed of both sleep stages 1 and 2. Sleep variables were not significantly different among the conditions. Subjective ratings of the nap time and sleep latency significantly differed among the conditions. In the Caffeine condition, subjectively estimated nap time was 3 – 5 min shorter and sleep latency was 3– 5 min longer than the other conditions. Furthermore, in the Caffeine condition, subjectively estimated nap time was significantly shorter than the total nap time measured by the polysomnogram [Fð1; 9Þ ¼ 14:94, p , 0:01].

Table 2 Sleep variables and subjective ratings of the nap (n ¼ 10) Condition

ANOVA

Nap

Face-washing

Bright-light

Caffeine

Fa

1

p

Sleep variables Time in bed Total sleep time Stage 1 Stage 2 Stages 3 þ 4 Stage REM Waking time after stage 1 onset Latency to stage 1

20.6 (4.0) 14.8 (0.4) 8.6 (3.5) 6.2 (3.6) – – 1.1 (1.5) 4.8 (3.0)

19.5 (2.3) 14.8 (0.8) 8.7 (3.6) 6.2 (3.8) – – 1.1 (0.4) 3.6 (2.1)

20.8 (3.9) 14.4 (1.4) 9.2 (2.6) 5.2 (2.8) – – 1.7 (1.8) 4.7 (3.0)

23.4 (4.8) 12.7 (4.7) 9.2 (4.7) 3.5 (3.8) – – 3.3 (5.4) 7.5 (7.2)

2.67 1.52 0.11 1.31

0.78 0.44 1.00 1.00

n.s. n.s. n.s. n.s.

1.17 2.11

0.48 0.57

n.s. n.s.

Subjective ratings Nap time Sleep latency Nap satisfactionb Mood after the napb

11.5 (5.7)* 5.4 (1.7)* 3.0 (1.4) 3.2 (0.9)

11.6 (4.2)* 4.1 (1.0)* 3.6 (1.4) 3.9 (0.6)

10.0 (4.7) 5.8 (3.1)* 2.8 (1.2) 4.0 (0.9)

3.49 6.71 1.93 1.89

0.92 0.57 0.91 0.90

,0.05 ,0.05 n.s. n.s.

* a b

Significantly different from the Caffeine condition (p , 0:05). Values in parentheses are S.D. df ¼ 3; 27. 5, Very good; 1, very poor.

6.4 9.3 2.4 3.4

(3.2) (4.7) (1.0) (1.1)

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Table 3 Summary of two-way [5ðconditionsÞ £ 5ðtime : blocks of 3 sessionsÞ] analysis of variance for repeated measures Condition (df ¼ 4, 36)a

Time (df ¼ 4, 36)a

Condition £ Time (df ¼ 16, 144)a

F

1

p

F

1

p

F

1

p

Subjective ratings Sleepiness Fatigue

7.17 1.46

0.96 0.86

, 0.001 n.s.

9.98 8.06

0.53 0.36

, 0.001 , 0.01

4.64 3.81

0.51 0.32

, 0.001 , 0.01

Performance measures Reaction time Percent miss

1.73 5.07

0.85 0.78

n.s. , 0.01

7.95 8.39

1.00 0.59

, 0.001 , 0.001

2.55 2.74

0.85 0.46

, 0.01 , 0.05

Physiological measures P3 amplitude EEG 4–11 Hz

0.68 4.69

1.00 0.79

n.s. , 0.01

6.56 0.41

0.99 0.45

, 0.001 n.s.

1.05 1.22

0.99 0.35

n.s. n.s.

a

The dfs in the P3 amplitude were 3, 27 for Condition and for Time, and 12, 108 for Condition £ Time.

3.2. Subjective and behavioral measures

(Table 5) after post-nap sessions were significantly greater than those of pre-nap sessions.

A summary of the ANOVAs in the subjective and physiological measures is shown in Table 3. Two-way [5ðconditionsÞ £ 5ðtime : blocks of 3 sessionsÞ] ANOVAs with repeated measures showed that the interactions of conditions by time were significant for subjective (sleepiness and fatigue) and behavioral measures (reaction time and percent miss). The mean values of subjective and behavioral measures averaged in the 3 sessions every 15 min are shown in Tables 4 and 5. The results of post hoc comparisons of conditions by time are also shown in these tables. 3.2.1. No-Nap condition In the No-Nap condition, subjective sleepiness and fatigue (Table 4), and reaction time and percent miss

3.2.2. Nap condition In the Nap condition, sleepiness for 15 –60 min after napping was significantly lower than the No-Nap condition. In comparison with pre-nap sessions, subjective sleepiness and fatigue, and percent miss were significantly higher during post-nap sessions. 3.2.3. Caffeine condition In the Caffeine condition, subjective sleepiness and fatigue for 15 – 60 min, reaction time for 30 – 60 min, and percent miss for 45 –60 min after napping were significantly lower than the Nap condition. Reaction time at 45– 60 min after napping was significantly shorter in the Caffeine condition than the Face-washing condition.

Table 4 Mean values in subjective ratings per 3 sessions (n ¼ 10) Condition

Pre-nap session 1–3

Post-nap sessions 1–3 (0 – 15 min)

4 –6 (15 –30 min)

7–9 (30 –45 min)

10 –12 (45 –60 min)

Sleepiness (0 –100) No-Nap Nap Face-washing Bright-light Caffeine

47.9 (22.3) 41.3 (16.6) 41.9 (17.5) 43.9 (24.1) 38.0 (18.0)

55.8 (23.0) 51.1 (21.2) 36.3 (13.2) 42.4 (27.9) 39.9 (20.9)

87.0 60.1 41.9 43.8 36.0

(14.9)a,c,d,e,f (26.1)b,f (23.4)b (36.0)b (21.2)b,c

85.9 65.8 50.8 45.6 38.1

(18.0)a,c,d,e,f (30.8)a,b,e,f (27.7)b (35.9)b,c (25.8)b,c

90.8 69.2 59.7 44.2 35.9

(13.9)a,c,d,e,f (32.8)a,b,e,f (26.6)b,f (36.1)b,c (29.3)b,c,d

Fatigue (0 – 100) No-Nap Nap Face-washing Bright-light Caffeine

28.8 (18.6) 31.4 (18.7) 26.0 (15.9) 28.7 (22.7) 31.6 (16.1)

38.0 (25.4) 38.2 (22.6) 29.6 (18.5) 29.6 (28.7) 35.6 (17.9)

47.2 42.7 33.4 37.0 31.8

(27.5)a,d,e,f (26.8)a,f (20.5)b (34.2)b (22.5)b,c

53.2 47.0 33.9 38.4 33.6

(32.1)a,d,e,f (33.2)a,d,e,f (19.3)b,c (34.7)b,c (24.0)b,c

57.6 52.0 41.1 36.5 33.7

(32.6)a,d,e,f (36.6)a,d,e,f (25.7)a,b,c (34.0)b,c (25.0)b,c

a b c d e f

Significant level was set at 5%. Values in parentheses are S.D. Significantly different from pre-nap sessions. Significantly different from the No-Nap condition. Significantly different from the Nap condition. Significantly different from the Face-washing condition. Significantly different from the Bright-light condition. Significantly different from the Caffeine condition.

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Table 5 Mean values in performance measures per 3 sessions (n ¼ 10) Condition

Reaction time (ms) No-Nap Nap Face-washing Bright-light Caffeine Percent miss No-Nap Nap Face-washing Bright-light Caffeine a b c d e f

Pre-nap session 1–3

453.8 (31.1) 469.4 (36.4) 467.8 (38.5) 468.6 (42.8) 469.6 (35.4) 0.6 (0.6) 1.2 (2.4) 0.4 (0.6) 0.8 (0.9) 0.4 (0.4)

Post-nap sessions 1–3 (0–15 min)

4 –6 (15–30 min)

7– 9 (30–45 min)

10–12 (45–60 min)

465.4 (40.2) 469.3 (39.9) 467.8 (44.5) 468.1 (43.0) 460.5 (42.8)

503.9 (46.7)a,f 480.1 (36.7) 480.8 (48.9) 484.9 (58.3) 460.5 (33.7)b

493.7 (51.7)a,f 489.3 (46.0)f 480.3 (60.0) 476.8 (46.0) 455.3 (35.8)b,c

494.2 (39.6)a,f 492.8 (45.2)f 489.4 (55.5)f 480.9 (53.4) 451.3 (31.9)b,c,d

4.2 (3.9)a,f 2.7 (3.9) 0.9 (0.7) 1.2 (1.8) 0.7 (0.6)b

7.2 (6.3)a,d,e,f 3.9 (5.5) 1.4 (1.3)b 3.0 (3.9)b 0.7 (0.9)b

7.8 4.4 2.1 3.1 1.1

(7.1)a,d,e,f (6.0) (2.1)b (3.9)b (1.6)b

6.6 5.7 3.3 1.7 1.0

(5.6)a,d,e,f (6.9)a,e,f (3.4)b (1.5)b,c (1.0)b,c

Significant level was set at 5%. Values in parentheses are S.D. Significantly different from pre-nap sessions. Significantly different from the No-Nap condition. Significantly different from the Nap condition. Significantly different from the Face-washing condition. Significantly different from the Bright-light condition. Significantly different from the Caffeine condition.

3.2.4. Conditions of Face-washing and Bright-light Subjective fatigue in the Face-washing condition was significantly lower than the Nap condition for 30 –60 min after napping. In the Bright-light condition, subjective sleepiness and fatigue for 30– 60 min and percent miss for 45 –60 min after napping were significantly lower than those of the Nap condition. There was no significant difference between the Face-washing and the Bright-light conditions.

performed. Neither the main effects nor interactions were significant, except for the interaction of condition by sessions in subjective sleepiness [Fð8; 72Þ ¼ 6:54, 1 ¼ 0:66, p , 0:001]. Table 7 shows the mean values of subjective sleepiness for 3 sessions immediately after the nap. Subjective sleepiness immediately after napping in the Nap condition was significantly higher than that in both the NoNap and Face-washing conditions.

3.3. Physiological measures

4. Discussion

The mean values of physiological measures averaged in the 3 sessions every 15 min are shown in Table 6. The ANOVAs showed that only the main effects of time in P3 amplitude or of condition in EEG 4 –11 Hz activity were significant (Table 3). P3 amplitude during post-nap sessions was significantly lower than pre-nap sessions. For EEG 4 – 11 Hz amplitude, one-way ANOVA (5 conditions) was performed within pre-nap or post-nap sessions. There were significant differences among the conditions during the post-nap sessions [Fð4; 36Þ ¼ 4:95, 1 ¼ 0:85, p , 0:01] but not during pre-nap sessions [Fð4; 36Þ ¼ 1:33, 1 ¼ 0:82, n.s.]. During the post-nap sessions, EEG 4– 11 Hz amplitude in the No-Nap condition was significantly higher than the other 4 conditions.

The main findings concerning the variables of nap, drug and action are summarized in Table 8. A short nap suppressed sleepiness and EEG 4 –11 Hz activity 15– 60 min after napping. However, it also induced sleepiness immediately after napping. Intake of caffeine and exposure to bright light further suppressed sleepiness and fatigue, and improved performance level. Face-washing suppressed sleepiness immediately after napping. These results suggest that the combination of a short nap and caffeine intake immediately before napping or actions (face washing or exposure to bright light) immediately after napping is a useful countermeasure to daytime sleepiness.

3.4. Effects on sleep inertia

Unless taking a nap (No-Nap condition), subjective mood (sleepiness and fatigue) and performance level (reaction time and percent miss) deteriorated. Moreover, EEG 4 –11 Hz band activity increased, compared with the other 4 conditions after taking a short nap. The subjects read

To examine the effects of sleep inertia immediately after the nap, two-way [5 conditions £ 3 sessions immediately after the nap] ANOVAs with repeated measures were

4.1. Effects of resting

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Table 6 Mean values in physiological measures per 3 sessions (n ¼ 10) Condition

Pre-nap session 1–3

Post-nap sessions 1–3 (0–15 min)

4–6 (15–30 min)

7–9 (30–45 min)

10– 12 (45–60 min)

Mean (0–60 min)

P3 amplitude (mV) at Pz area Nap Face-washing Bright-light Caffeine Mean

13.8 (3.8) 14.1 (6.3) 14.0 (4.7) 13.1 (5.1) 13.8 (4.3)

9.3 (4.8) 10.5 (6.4) 11.3 (5.2) 11.8 (6.0) 10.7 (4.4)a

10.6 (5.0) 12.1 (7.4) 11.2 (5.1) 12.1 (5.0) 11.5 (4.8)a

9.7 (4.2) 12.2 (7.5) 11.5 (5.8) 12.6 (4.7) 11.5 (4.9)a

10.4 (4.0) 11.8 (6.1) 11.8 (5.9) 13.0 (4.7) 11.8 (4.5)a

10.0 (4.0) 11.6 (6.7) 11.4 (5.3) 12.4 (4.8)

EEG 4– 11 Hz band (mV) at C3 area No-Nap Nap Face-washing Bright-light Caffeine Mean

13.2 (2.5) 11.7 (1.7) 12.3 (2.1) 12.8 (2.4) 12.4 (2.0) 12.5 (1.7)

13.8 (2.9) 11.3 (2.0) 12.5 (2.7) 12.8 (1.0) 10.8 (1.7) 12.2 (1.7)

14.5 (3.2) 11.6 (1.9) 11.9 (2.3) 12.3 (1.5) 11.1 (2.0) 12.3 (1.6)

14.9 (5.1) 12.3 (2.7) 12.4 (2.9) 12.1 (1.6) 11.3 (1.9) 12.6 (1.9)

15.0 (4.9) 11.2 (2.2) 13.5 (4.9) 12.2 (1.0) 11.5 (2.0) 12.7 (2.4)

14.6 (3.6)c,d,e,f 11.6 (1.9)b 12.6 (3.0)b 12.4 (1.2)b 11.2 (1.9)b

a b c d e f

Significant level was set at 5%. Values in parentheses are S.D. Significantly different from pre-nap sessions. Significantly different from No-Nap condition. Significantly different from the Nap condition. Significantly different from the Face-washing condition. Significantly different from the Bright-light condition. Significantly different from the Caffeine condition.

a newspaper for 20 min during the rest period in the No-Nap condition, sitting in a semi-reclining chair. The possibility was not completely denied that this de-activating manipulation caused a lowered alertness after the rest. If this were true, sleepiness would markedly increase immediately after the rest. However, it was not the case for the first post-rest session for the No-Nap condition. In fact, sleepiness decreased 8.0 points in the first post-rest session in comparison with the pre-resting sessions (Table 7). Therefore, other factors such as post-lunch dip would affect the results more than the de-activating manipulation by sitting Table 7 Subjective sleepiness on the first 3 post-nap sessions (n ¼ 10) Condition

No-Nap Nap Face-washing Bright-light Caffeine

Post-nap sessions 1

2

3

28.0 (25.4)d 10.1 (17.5)c,e 26.6 (11.9)d 3.7 (26.7) 5.7 (12.4)

7.7 (28.4)a 7.5 (18.2) 25.4 (12.4) 24.2 (24.6) 0.1 (12.8)

23.9 (27.9)a,b,d,e,f,g 11.9 (21.6)c,e 24.9 (15.4)c,d 24.1 (25.9)c 0.0 (11.2)c

The values were transformed by subtracting the mean values of the 3 pre-nap sessions as a baseline. Values in parentheses are S.D. a Significantly different from the first session. b Significantly different from the second session. c Significantly different from the No-Nap condition. d Significantly different from the Nap condition. e Significantly different from the Face-washing condition. f Significantly different from the Bright-light condition. g Significantly different from the Caffeine condition.

in a semi-reclining chair. A short break during lunchtime might be less effective on afternoon sleepiness. 4.2. Effects of a short nap When taking a short nap (Nap condition), subjective sleepiness and EEG 4 –11 Hz band activity were suppressed. These results suggest that a short nap improved alertness, supporting our previous findings that showed positive effects of a daytime short nap (Hayashi et al., 1999a,b, 2003; Tamaki et al., 1999, 2000). However, task performance was not significantly different between the Nap and No-Nap conditions. These results also supported our previous findings that a short nap taken in the mid-afternoon (14:00 hours) improved both subjective sleepiness and task performance (Hayashi et al., 1999b) while a short nap taken at noon only improved subjective sleepiness (Hayashi et al., 1999a, 2003). However, it should be noted that subjective sleepiness and fatigue during post-nap sessions increased in comparison with the pre-nap sessions. These suggest that this nap did not completely restore subjective mood. Therefore, the combination of a nap and other countermeasures would be required for practical use. In the present study, as in previous ones (Hayashi et al., 1999a,b; Stampi et al., 1990), short naps mainly consisted of lighter sleep (sleep stages 1 and 2). A longer nap of greater than 30 min during the daytime is relatively counterproductive because more severe sleep inertia would occur by waking from deeper sleep stages (Stampi et al., 1990; Tietzel and Lack, 2001). However, shorter and lighter sleep

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Table 8 Summary of the main results Variables

Conditions

Measures and post-nap time being significant effects

Sleepiness immediately after napping

Sleep

Nap (versus No-Nap)

Increased

Drug

Caffeine (versus Nap)

Action

Face-washing (versus Nap) Bright-light (versus Nap)

Sleepiness decreased for 15–60 min EEG 4–11 Hz attenuated for post-nap sessions Sleepiness and fatigue decreased for 15 –60 min Reaction time shortened for 30– 60 min Percent miss decreased for 45–60 min Fatigue decreased for 30–60 min Sleepiness and fatigue decreased for 30 –60 min Percent miss decreased for 45–60 min

inertia does occur in such a short nap (Hayashi et al., 2003). In the present findings, subjective sleepiness immediately after the nap was significantly higher in the Nap condition compared with the No-Nap condition. These results suggest that sleep inertia occurred within 5 min after the nap in the Nap condition. 4.3. Effects of caffeine In the present findings, the combination of caffeine and a short nap was the most effective among all conditions. These results are consistent with the results of Reyner and Horne (1997). They showed that the combination of caffeine and a short nap was clearly superior to caffeine alone or taking a nap alone. Such combination effects of naps and caffeine were also confirmed in the 24 h of sustained performance (Bonnet and Arand, 1994). Although the effect of caffeine alone was not tested in the present study, Horne and Reyner (1996) reported that the effects of a short nap alone were comparable with those of caffeine alone. One cup of coffee contains approximately 60 –80 mg of caffeine (Brice and Smith, 2002; Mandel, 2002); so the 200 mg caffeine used in the present study corresponds to approximately 3 cups of coffee. This dosage of caffeine is somewhat high for practical use. Although Horne and Reyner (1997) reported that the alerting effects of 150 mg caffeine were similar to those of 200 mg, the effects of caffeine are generally dose-dependent (Kaplan et al., 1997; Bonnet et al., 1995). Further studies on the effects of lower doses of caffeine and a short nap may be required. In the present study, fresh cream was added to the coffee to mask the bitter taste of caffeine. The effects of caffeine would be modified by the addition of fresh cream. Quinlan et al. (1997) reported that addition of milk to coffee or tea did not alter salivary caffeine levels, however, it moderated activation of the sympathetic nervous system immediately (0 –3 min) after taking the beverages. In addition, milk addition reduced anxiety and improved mood 30– 60 min after intake. Although the extent to which fresh cream counteracted the effects of caffeine was not clear in the present study, the alerting effects of caffeine were still observed.

No significant difference

Decreased No significant difference

4.4. Effects of bright light In the present study, an exposure to bright light for 1 min immediately after a short nap lowered subjective sleepiness and fatigue, and reduced the percent miss in the memory search task. These findings could be comparable with the combination of caffeine and a short nap except for the reaction time of the task. This is interesting because many studies hardly confirmed the effects of bright light during the daytime despite using a much longer duration of exposure to bright light greater than 1.5 h (Badia et al., 1991; Daurat et al., 1993; Lafrance et al., 1998; Leproult et al., 2001). In contrast, several studies observed the positive effects of bright light during daytime in psychological and psychophysiological measures (Gru¨nberger et al., 1993; Saito et al., 1996). Gru¨nberger et al. (1993) observed that exposure to 2500 lx of bright light for 4 h during daytime between 09:00 and 17:00 hours improved noo- and thymopsyche and psychophysiological measures in the afternoon. Furthermore, Saito et al. (1996) found that exposure to bright light (5000 lx) for 20 min in the morning between 10:00 and 12:00 hours enhanced muscle sympathetic nerve activity and heart rate. In these studies (Gru¨nberger et al., 1993; Saito et al., 1996), similar to the present study, the subjects took usual nocturnal sleep and were not sleep deprived or sleep restricted from the prior nocturnal sleep. These findings suggest that bright light for a short duration might be useful for preventing sleep inertia from a short daytime nap or post-lunch dip following a normal nocturnal sleep, where sleepiness or fatigue were not severe. Exposure to bright light during the morning or evening advanced or delayed the circadian phase of body temperature, respectively (Carrier and Dumont, 1995; Czeisler et al., 1989; Dumont and Carrier, 1997). Moreover, bright light suppresses the secretion of melatonin and prevents the decline of body temperature during the night (Badia et al., 1991; Campbell and Dawson, 1990; Daurat et al., 1993; Dawson and Campbell, 1991; Myers and Badia, 1993; Cajochen et al., 2000). These previous and the present findings suggest that bright light has not only a chronobiological effect but also a directly alerting stimulation effect.

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4.5. Effects of face washing Face washing immediately after the nap suppressed subjective sleepiness immediately after napping. However, this effect did not last for 60 min after the nap. In comparison with taking a nap alone, the combination of a short nap with face washing reduced fatigue between 30 and 60 min after the nap. These results suggest that face washing added mild and transient effects to the effects of a daytime short nap. These results support previous findings that face washing using cold water was a simple but effective tool to combat sleep inertia (Ferrara and De Gennarro, 2000), and cold air had marginal and transient effects for car drivers (Reyner and Horne, 1998).

5. Conclusions The present findings confirmed that the daytime short nap had positive effects on the decline in alertness during the mid-afternoon, i.e., post-lunch dip, and that the effects of such a nap could be enhanced by caffeine intake before taking the nap, by exposure to bright light or face washing immediately after waking from the nap. In the present study, the combination of caffeine, bright light and face washing was not examined. It was reported that the combination of bright light and caffeine reduced melatonin secretion and enhanced body temperature, alertness and performance after prolonged sleep deprivation (Wright et al., 1997, 2000). Further effects might be expected when combining all these countermeasures on the post-lunch dip for practical use. Finally, the present subjects were young adults who did not have sleep –wake problems. The present strategies may not always be useful for other age groups. For example, some elderly people reported that they could not take a nap after drinking a cup of coffee. Moreover, the present subjects were not sleep restricted during the experimental period. Therefore, further studies are needed to clarify whether the present strategies could be useful for other age groups or for those who have sleep –wake problems.

Acknowledgements This study was performed through Special Coordination Funds for Promoting Science and Technology from the Ministry of Education, Culture, Sports, Science and Technology, The Japanese Government.

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