The influence of morningness–eveningness on anxiety and cardiovascular responses to stress

Physiology & Behavior 85 (2005) 125 – 133

The influence of morningness–eveningness on anxiety and cardiovascular responses to stress Thomas A. Willis*, Daryl B. O’Connor, Lawrence Smith Institute of Psychological Sciences, University of Leeds, Leeds LS2 9JT, UK Received 14 September 2004; received in revised form 15 March 2005; accepted 21 March 2005

Abstract The relationship between cardiovascular responses to stress and health outcomes is inconsistent. In this study, the effects of morningness – eveningness and time of day upon cardiovascular activity at rest and in response to stress were examined. Sixty morning-types and eveningtypes completed two testing sessions (one morning, one afternoon) that comprised a battery of three stress tasks and a measure of anxiety. The results failed to support a time of day effect upon cardiovascular activity, but there was evidence of an interaction between time of day and morningness – eveningness upon heart rate (HR) and rate pressure product (RPP; HR  SBP). Evening-types exhibited higher HR and RPP in the afternoon, both at rest and during stress. A time of day effect was shown for mood, with anxiety levels higher in the morning than the afternoon. These results are discussed in terms of their health and methodological implications. D 2005 Elsevier Inc. All rights reserved. Keywords: Anxiety; Cardiovascular reactivity; Gender; Morningness – eveningness; Psychological stress; Time of day

1. Introduction Since its proposal, the Freactivity hypothesis_ [1] has received considerable research attention in the fields of stress medicine and psychophysiology [2,3]. The central tenet of the hypothesis is that individuals displaying exaggerated emotional and physiological responses to stress may be more likely to develop health problems in the future. More specifically, those individuals prone to display dramatic increases in heart rate and/or blood pressure after stress exposure could be at greater risk of developing future hypertension and other cardiovascular complications. Strong supportive evidence for this hypothesis comes from animal studies [e.g. 4], as well as a wealth of examples from human research. For instance, results from the Kuopio Heart study in Finland have consistently indicated that heightened cardiovascular activity due to the anticipation of imminent strenuous exercise is associated with prospective health difficulties including hypertension [5] and stroke [6]. * Corresponding author. Tel.: +44 113 343 5739; fax: +44 113 343 5749. E-mail address: [email protected] (T.A. Willis). 0031-9384/$ - see front matter D 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.physbeh.2005.03.013

Furthermore, Matthews et al. [7] have demonstrated that the cardiovascular response to stress can predict blood pressure levels at 13 year follow-up. Not all studies have been supportive, however, and some authors argue that reactivity adds no further prognostic information than can be obtained from measuring resting blood pressure [e.g. 8]. Various explanations have been offered for these inconsistencies, including the choice of stressor and the extent to which the stress tasks contain social and evaluatory aspects [see e.g. 2,9]. Two factors that have received considerably less attention, however, are the time of day that testing occurs and the extent to which the participants are morning-types or evening-types. It is well established that biological rhythms fluctuate over the course of the day. In normal circumstances, blood pressure and heart rate, for example, are known to rise during the day and decrease during the night [10]. Furthermore, circadian patterns are also evident in the occurrence of cardiovascular events, such as myocardial infarction (MI) and stroke [e.g. 10,11]. In particular, these occur most frequently in the morning hours, with a second, smaller peak in the early evening. With this in mind, research has considered whether

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humans’ cardiovascular systems are more reactive to stress at certain times of the day than at other times. Nebel et al. [12] proposed that cardiovascular reactivity to stress may vary depending upon the time that it is measured. If reactivity were to rise and peak in the morning hours (as do heart rate and blood pressure), this could provide an explanation as to the increased incidence of cardiovascular events at this time. However, many of the recent studies of the Freactivity hypothesis_ and of the stress response more generally have failed to control for this important and potentially confounding variable [e.g. 13-15]. A second possible confounding variable that has received little or no attention is the individual differences variable morningness– eveningness (also known as chronotype). It has long been known that humans vary in their preference for the timing of activities such as exercise or going to bed, as well as their ability to perform certain tasks during the day. Early work by Kleitman [16] classed people as Flarks_ or Fowls_ depending on these preferences, and argued that Flarks_ (or Fmorning-types_) function better in the morning, performing more successfully on various cognitive tasks. FOwls_ (or Fevening-types_), meanwhile, have an optimal level of alertness in the afternoon and early evening. This individual difference is often referred to as Fmorningness– eveningness_ [17]. Moreover, morning and evening types have been found to differ in their circadian rhythms of body temperature, catecholamine excretion and cortisol secretion [e.g. 18,19]. Specifically, the rhythms of evening types tend to display a phase delay (i.e. they reach their peak later in the day) relative to morning types. Surprisingly, to the best of our knowledge, only one study has examined the potential moderating role of morningness – eveningness within the context of the Freactivity hypothesis_ and stress research per se [12]. Nebel and colleagues [12], in a small, male-only sample found a significant interaction between participants’ level of morningness– eveningness (i.e. whether they were a morning or an evening type) and the time of testing. Morning types showed greater absolute levels and reactivity in the morning session, while evening types showed greater levels in the afternoon. There was no support, however, for a simple time of day effect. Although this may have been masked by the small sample assessed (n = 19 in their first study and n = 11 in their second). In addition, there was some evidence for an interactive effect of morningness– eveningness and time of day upon mood. Akin to the physiological data, morning-types reported greater negative affect in the morning than the afternoon, while this pattern was reversed in the evening-types. However, this pattern was only apparent in their second sample. Therefore, given the paucity of studies that have examined the role of morningness– eveningness in stress research, this study sets out to investigate further the influence of morningness – eveningness in a sample of men and women. While gender differences tend not to be shown in self-reported morningness –eveningness [see e.g.

20], they have emerged in studies of cardiovascular responses to stress. At rest, males tend to display greater SBP while females show a higher HR [e.g. 21]. In terms of levels in response to stress, while there is some inconsistency [22], there is some evidence that males tend to show greater changes in blood pressure, while females show greater heart rate increases [e.g. 23]. Therefore, the influence of gender on stress reactivity will also be examined in the current study. In light of the work reviewed, this study set out to investigate the extent to which morningness –eveningness and the time of day that testing occurs influences cardiovascular levels (blood pressure and heart rate) at rest and in response to psychological stress. The potential effects upon anxiety are also investigated. Specifically, we hypothesised that: 1. Cardiovascular levels at rest and in response to psychological stress will be significantly greater in the morning session compared to the afternoon session (consistent with the incidence of cardiovascular events findings). 2. Morning-types tested in the morning session will exhibit significantly greater cardiovascular levels at rest and in response to psychological stress compared to during the afternoon session. 3. Evening-types tested in the afternoon session will exhibit significantly greater cardiovascular levels at rest and in response to psychological stress compared to during the morning session. 4. Morning- and evening-types will exhibit significantly different levels of anxiety during the morning and afternoon testing sessions. 5. Gender will have a significant effect on cardiovascular levels at rest and in response to psychological stress.

2. Method 2.1. Participants Participants ranged in age from 18 to 35 years, with a mean age of 21.9 T 4.21 years. A cohort of 60 participants (32 male, 28 female) was recruited from a larger sample (N = 1213) of undergraduate and postgraduate students at a university in the north of England who completed the Composite Scale of Morningness (CSM) [24]. The scale was distributed in lectures and cafeteria, while a website was created to collect online responses. The 60 participants were selected on the basis of their CSM score (see Measurement of morningness –eveningness, below) as well as meeting the inclusion criteria (non-smoker; no history of heart condition, hypertension, diabetes, depression and/or any psychiatric disorder; not currently taking any medication that may affect cardiovascular activity [e.g. betablockers]). All those recruited to the study received a U10 cash payment after completing the study.

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2.2. Design A repeated measures design was utilised, with all participants completing two laboratory testing sessions, between 8 and 9 am, and between 1 and 2 pm. In general, the two testing sessions took place 1 or 2 days apart (median = 1 day, range = 0 – 14 days). The order of the sessions was counterbalanced between subjects and the researchers were blind to participants’ level of morningness – eveningness during all sessions. All tasks (see Stress tasks below) were presented to participants using standardised instructions. No training or practice trials took place to maximize the novelty and stressful nature of the tasks. The order of the mental arithmetic task and mirror trace task was randomised between subjects, with the social speaking task always presented last. Participants were asked to avoid caffeinated drinks and strenuous physical exercise for at least 1 h prior to each testing session. They were also asked to have eaten prior to arrival. 2.3. Measurement of morningness –eveningness The 13-item CSM comprises items from the two scales of Horne and Ostberg [25] and Torsvall and Akerstedt [26], and has been shown to possess superior psychometric properties than the scales from which the items are derived [24]. Greenwood [27] replicated the finding of Smith et al. [24] that the CSM possessed good psychometric properties and also reported that CSM scores were stable over time and did not change even when respondents were exposed to night- and shiftworking conditions. Both Duffy, Rimmer and Czeisler [28] and Griefahn, Kunemund, Golka, Thier, and Degan [29] have demonstrated that an individual’s level of morningness –eveningness, as identified by the Horne and Ostberg [25] scale, is a reliable indicator of chronotype. CSM items include FHow alert do you feel during the first half hour after having awakened in the morning?_ FNot at all alert_, FSlightly alert_, FFairly alert_ or FVery alert_; and FAt what time in the evening do you feel tired and, as a result, in need of sleep?_ F20.00– 21.00 pm_, F21.00 –22.15 pm_, F22.15 pm –00.30 am_, F00.30 – 01.45 am_, F01.45 – 03.00 am_. All items were rated on a 4- or 5-point scale, the coding of which differed by question. Scores range from 13 (extreme eveningness) to 55 (extreme morningness). In our original sample of 1213 participants, the scores were normally distributed and ranged from 14 to 52. Therefore, in order to recruit individuals towards the extremes of the distribution, initially respondents who scored within the upper and lower quartiles of the distribution were invited to participate in the second part of the study. However, in order to recruit sufficient numbers (approx. 30 per group) and to maintain a Ffull quartile gap_ between the groups, the inclusion criteria were shifted. In the final sample, participants scoring below 28 were classed FE-types_ (N = 32;

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mean CSM score = 24.06 (SD = 3.71)), and those above 33 as FM-types_ (N = 28; mean CSM score = 38.96 (SD = 3.50)). In addition, as a manipulation check, the E-types significantly differed from the M-types on the CSM scale (t = 15.94, df = 58, p < 0.001) indicating that the samples varied in their usual sleep/wake habits and preference for performing various activities. From the initial sample, 31.5% of respondents scored 28 or below, 43.5% scored 33 or above, leaving 25% in the centre of the distribution. Internal reliability for this scale in this sample was good (Cronbach’s alpha = 91). 2.4. Stress tasks 2.4.1. Mental arithmetic task (MA) Participants were presented with a 4-digit number, from which they were asked to continually subtract 13, for a total of 4 min. If/when the participant gave an incorrect response, they were asked to begin again from the initial number. A different 4-digit starting number was presented at the second session. The experimenter prompted the participant to perform more quickly at certain intervals (approx. after 1 min 30 s and 2 min 30 s). Cardiovascular data (systolic blood pressure [SBP], diastolic blood pressure [DBP] and heart rate [HR]) were collected by inflating the blood pressure cuff after 30 s, 1 min 45 s and 3 min. Serial subtraction tasks similar to this have been successfully utilised in several previous reactivity studies [e.g. 30-32]. 2.4.2. Mirror trace task (MT) Participants were asked to trace around a 5-point star, which they could only see in a mirror, for a total of 4 min. They were required to reverse their direction around the star after every completion, and asked to perform as quickly and as accurately as possible. The experimenter prompted the participant to perform more quickly and more accurately at certain intervals (approx. after 1 min 30 s and 2 min 30 s). Cardiovascular data were collected after 30 s, 1 min 45 and 3 min. In the second session, the star was rotated 180-. The mirror trace task has featured in many reactivity studies, producing generally consistent results [e.g. 33-35]. 2.4.3. Social speaking (SS) In their first testing session, participants were given a list of nine topics (including euthanasia, vivisection and fox hunting, etc) and asked to prepare a presentation about one of them. In the second session, a social stressor based upon Matthews et al. [15] was utilised. The participant was given a scenario where they had been accused of stealing a wallet that they had found on the floor and was asked to present a speech defending themselves. The testing room possessed a one-way mirror and participants were told that their presentation would be filmed using a video camera situated behind this mirror. They were given 3 min to prepare, and then asked to speak for a further 3 min. If the presentation finished before the allotted time was up, they were asked to

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Table 1 Mean (SD) levels and change scores for all cardiovascular variables Cardiovascular AM session measures M-types (N = 28)

PM session E-types (N = 32)

M-types

E-types

Baseline SBP (mm Hg) DBP (mm Hg) HR (bpm) RPP (mm Hg*bpm)

118 T 10 72 T 8 76 T 11 9034 T 1654

122 T 10 73 T 7 75 T 13 9137 T 1791

119 T 8 71 T 8 76 T 10 9002 T 1510

123 T 11 72 T 9 80 T 12 9837 T 1728

Pre-MA SBP (mm Hg) DBP (mm Hg) HR (bpm) RPP (mm Hg*bpm)

119 T 10 73 T 7 76 T 10 9054 T 1638

122 T 10 73 T 8 75 T 13 9153 T 1811

119 T 9 71 T 9 75 T 10 8963 T 1595

123 T 11 74 T 13 80 T 12 9858 T 1737

MA SBP (mm Hg) 129 T 14 130 T 13 128 T 13 133 T 10 Change score 10 T 10 9T8 10 T 8 9T6 DBP (mm Hg) 80 T 10 81 T10 80 T 11 81 T 8 Change score 7T9 8T6 9T7 7 T 11 HR (bpm) 84 T 13 85 T 15 82 T 13 87 T 14 Change score 8T8 10 T 8 7T6 7T8 RPP (mm 10,901 T 2661 11,103 T 2505 10,564 T 2356 11,533 T 2078 Hg*bpm) Change score 1847 T 1809 1951 T1492 1600 T 1306 1675 T 1298 Pre-MT SBP (mm Hg) DBP (mm Hg) HR (bpm) RPP (mm Hg*bpm)

120 T 10 72 T 9 75 T 10 8966 T 1544

121 T 9 74 T 7 74 T 12 8992 T 1496

121 T10 71 T 9 74 T 9 8950 T 1578

124 T 10 73 T 8 78 T 10 9708 T 1375

MT SBP (mm Hg) 130 T 10 129 T 13 129 T 11 130 T 11 Change score 10 T 7 8T9 8T5 6T8 DBP (mm Hg) 81 T 9 82 T 10 81 T 8 79 T 9 Change score 8T7 8T7 10 T 5 6T5 HR (bpm) 78 T 13 77 T 15 78 T 11 80 T 12 Change score 3T6 2T9 4T6 2T6 RPP (mm 10,129 T 2117 10,000 T 2650 10,039 T 1949 10,400 T 1873 Hg*bpm) Change score 1163 T 1330 1008 T 1787 1089 T 960 692 T 1127 Pre-SS SBP (mm Hg) DBP (mm Hg) HR (bpm) RPP (mm Hg*bpm)

121 T10 73 T 9 72 T 11 8759 T 1719

122 T 9 76 T 8 74 T 10 9051 T1281

119 T 10 72 T 10 72 T 9 8635 T 1515

122 T 9 72 T 8 76 T 10 9322 T 1343

SS SBP (mm Hg) 132 T 11 132 T 11 131 T11 133 T 11 Change score 12 T 8 10 T 8 12 T 6 11 T 7 DBP (mm Hg) 83 T 9 83 T 9 83 T 9 83 T 9 Change score 10 T 8 7T6 10 T 6 11 T 6 HR (bpm) 80 T 10 81 T14 80 T 12 85 T 14 Change score 8T8 6 T 10 7T6 9 T 10 RPP (mm 10,683 T 1854 10,674 T 2330 10,547 T 2319 11,370 T 2459 Hg*bpm) Change score 1924 T 1435 1624 T 1888 1911 T1234 2048 T 1724

continue speaking. If they stopped again before the task was completed, they were asked a question referring to part of their presentation. During both sections, the blood pressure cuff was inflated after 30 s and 2 min. 2.4.4. Ambulatory blood pressure monitoring All cardiovascular data were collected using a SpaceLabs 90207 ambulatory blood pressure monitor and appropriately sized cuff. This has been used successfully in other studies [e.g. 36]. Heart rate and blood pressure measures were averaged across periods to provide mean heart rate (HR), systolic blood pressure (SBP) and diastolic blood pressure (DBP) for each rest and task period. Rate pressure product (RPP; SBP  HR) was also calculated at each point as an estimate of myocardial demand. Cardiovascular change scores were calculated by subtracting mean pretask resting levels from the subsequent mean task level. The baseline monitoring period consisted of 15-min seated rest. The cuff was fitted to the participants’ nondominant arm and the monitor was activated after 5, 10, 12 and 14 min. The first of these was discarded, and the baseline figure was calculated as the average of the remaining 3 measurements (based on the procedure of [13]). During the tasks, the blood pressure monitor was activated at the intervals outlined in the preceding section. During the rest periods, measurements were collected at 2min intervals (following [37]). 2.5. Questionnaire measures All participants also completed a short demographic questionnaire, which collected, among other information, self-estimated height and weight to allow the calculation of body mass index (BMI: kg/m2). At the start and end of both sessions, participants completed the 6-item state anxiety scale [38]. Items included FI feel calm_ and FI am worried_. All items were rated on a 4-point scale, extending from FNot at all_ to FVery much_. Reliability and validity data are reported by Marteau and Bekker [38]. Internal reliability for this scale with this sample was good (all Cronbach’s alphas >.71). 2.6. Procedure Participants were welcomed to the testing room, which was warm, quiet and well-lit. They were seated and fitted Notes to Table 1: Key: SBP: Systolic blood pressure MA: Mental arithmetic DBP: Diastolic blood pressure MT: Mirror trace HR: Heart rate SS: Social speaking RPP: Rate pressure product (SBP  HR)

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with the blood pressure cuff. They then spent a short time completing questionnaires and scales. Although it was not inflated and no measurements taken during this period, the cuff was fitted at the start of the session to allow participants to become accustomed to wearing it. Once the questionnaires and scales had been completed, the testing session began. Each session comprised a 15-min seated resting baseline period, followed by the three tasks, which were separated by 10-min rest periods. A further 10-min rest period followed completion of the final task.

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numbers of males and females (E-types: 19 m, 13 f; Mtypes: 13 m, 15 f). A 2  2 ANOVA testing task efficacy revealed significant increases between pretask rest and task for SBP ( F(1,59) = 233.71, p < 0.001), DBP ( F(1,59) = 257.55, p < 0.001), HR ( F(1, 59) = 85.94, p < 0.001) and RPP ( F(1,59) = 137.02, p < 0.001). Bonferroni-corrected paired t-tests showed that all three tasks produced significant increases in cardiovascular activity (all p < 0.01). 3.1. Cardiovascular levels at rest

2.7. Statistical analyses One-way analyses of variance (ANOVA) were used to investigate whether the groups significantly differed in their BMI. Further preliminary analyses assessed the efficacy of the stress tasks using a series of 2  2 (Time [a.m./ p.m.]  task/rest) ANOVAs. Absolute cardiovascular levels were analysed using a series of 3  2  2  2 repeated measures ANOVA). Task (MA, MT, SS) and Time (a.m./p.m.) were analysed as within-subjects factors, with morningness –eveningness (M/ E-type) as a between-subjects factor. Gender was also included as a between-subjects factor in all analyses. Cardiovascular levels were analysed in two ways: levels at rest (with baseline cardiovascular activity as the dependent variable [DV], and levels in response to stress (with average task activity as the DV). Cardiovascular reactivity was analysed using 3  2  2  2 analyses of covariance (ANCOVA). Change scores were the dependent variable, and average rest levels were included as covariates. Self-reported anxiety was analysed in the same way as cardiovascular levels, using mood scores from the beginning of both sessions.

3. Results Descriptive statistics for all study variables are shown in Table 1. No significant differences were found in BMI between morning and evening types ( F(1,56) = 0.64, p > 0.05) or between males and females ( F(1,56) = 0.83, p > 0.05). The two groups comprised approximately equal

For cardiovascular levels at rest, the results of the analysis revealed a main effect of gender for SBP and HR. Males were found to have significantly higher SBP than females ( F(1,56) = 4.09, p < 0.05; 122.56 mm Hg and 117.8 mm Hg, respectively) while females displayed significantly higher HR than males ( F(1, 56) = 14.42 p < 0.01; 81.64 bpm and 72.53 bpm, respectively). No significant main effects for time of testing or morningness– eveningness were found. However, strong trends for an interaction between time of testing and morningness – eveningness for HR ( F(1,56) = 3.96, p = 0.051) and RPP (F(1,56) = 3.92, p = 0.053) were observed. Inspection of the means indicated that this effect was due to greater HR and RPP at rest in the evening-types in the afternoon compared to the morning session. Moreover, when these analyses were conducted without gender, the time of testing  morningmorningness– eveningness interactions were both statistically significant (HR: F(1,58) = 4.03, p < 0.05); RPP: F(1,58) = 4.13, p < 0.05; see Fig. 1). 3.2. Cardiovascular levels in response to stress ANOVA revealed no significant main effects for time of testing or for morningness –eveningness for any of the cardiovascular variables. However, a significant time of testing  morningness – eveningness interaction was found for HR ( F(1,56) = 4.58, p < 0.05) and RPP (F(1,56) = 5.13, p < 0.05). Inspection of the means indicated that this interaction was due to the evening types and their greater HR in response to stress in the afternoon session relative to the morning (see Fig. 2). In addition, a significant main 10500

84

RPP (SBP * HR)

82

HR (bpm)

80 78 76 74 72 70 68

10000 9500 9000 8500 8000 7500

AM

PM

Time of day E-type

M-type

AM

PM

Time of day E-type

M-type

Fig. 1. The effect of morningness – eveningness and time of day on resting HR (left panel) and RPP (right panel).

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T.A. Willis et al. / Physiology & Behavior 85 (2005) 125 – 133 12000

88

RPP (SBP * HR)

86

HR (bpm)

84 82 80 78 76 74

11500 11000 10500 10000 9500 9000

72 AM

PM

AM

PM

Time of day

Time of day

E-type

E-type

M-type

M-type

Fig. 2. The effect of morningness – eveningness and time of day on HR (left panel) and RPP (right panel) in response to stress.

effect of gender for HR was found with females again displaying greater HR than males ( F(1, 56) = 3.35, p < 0.005; 86.27 bpm and 77.05 bpm, respectively). Females were also found to display lower SBP across all tasks compared to males, although this effect just missed statistical significance ( F(1,56) = 3.35, p = 0.073). A significant effect for Task was shown for all cardiovascular variables (SBP: F(2,112) = 6.45, p < 0.005; DBP: F(2, 112) = 10.13, p < 0.001; HR: F(2,112) = 27.02, p < 0.001; RPP: F(2,112) = 16.22, p < 0.001). Although there was variation in the levels reached during each task, the levels reached during social speaking were significantly higher than those reached during mental arithmetic for all the cardiovascular outcomes. 3.3. Cardiovascular change scores For cardiovascular reactivity levels, ANCOVA revealed no significant main effects for time of testing or morningness –eveningness, nor for the time of testing  morningmorningness – eveningness interaction on any of the cardiovascular variables. However, a significant time of testing  task interaction emerged for RPP (F(2,108) = 4.69, p < 0.05). Inspection of the means indicated that this effect was due to a combination of increased RPP reactivity to the mental arithmetic task in the morning session, and to RPP reactivity to the social stressor task in the afternoon session. 3.4. Self-reported anxiety A 2  2  2 repeated measures ANOVA revealed a significant main effect of time of testing on self-reported 10.5

Anxiety

10 9.5 9 8.5 8 AM

PM

Time of day

Fig. 3. The effect of time of day on self-reported anxiety.

anxiety (F(1,55) = 5.4, p < 0.05). Anxiety levels were found to be significantly higher in the morning session than the afternoon (see Fig. 3). The main effect of morningness– eveningness and the time of testing  morningness– eveningness interaction were non-significant.

4. Discussion This study provided evidence that individual differences in Fmorningness – eveningness_ may have a significant influence on cardiovascular activity. Groups of morningand evening-types were tested at two times of the day and their cardiovascular activity, both at rest and in response to stress, was recorded. It was hypothesised that a time of day effect would be apparent in the whole sample, with cardiovascular levels and reactivity being higher in the morning compared to the afternoon session (following the observed pattern for the incidence of cardiovascular events). Secondly, an interaction was proposed between morningness– eveningness and time of day. Morning-types were hypothesised to display greater cardiovascular activity in the morning session than the afternoon, while evening-types would show greater cardiovascular activity in the afternoon than the morning. A final hypothesis predicted that gender would have a significant effect upon cardiovascular activity, both at rest and in response to stress. While finding no evidence supporting the hypothesis that cardiovascular levels would be significantly greater in the morning session compared to the afternoon, the current study did find an interaction between time of testing and morningness – eveningness upon cardiovascular activity. Significant effects were shown for HR and RPP, with evening-types displaying significantly greater cardiac activity in the afternoon session than in the morning session. This pattern was evident both at baseline and in the absolute levels reached during stress. This is consistent with the results of Nebel et al., [12] who found an interaction between morningness – eveningness and time of day on absolute levels of SBP, HR and RPP. They also found some evidence of an interaction for cardiovascular change scores but the current study failed to replicate this effect.

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Our results demonstrate that morningness– eveningness has a significant effect on cardiovascular activity, and that its influence is not straightforward—indeed, no main effect of the variable emerged in any of the analyses. These data suggest that the effects of morningness– eveningness may only be observed in conjunction with time of day. This is an important finding and may have implications for existing and future stress research. For example, as noted earlier, there remain marked inconsistencies in the cardiovascular reactivity literature and it is possible that a failure to account for morningness– eveningness (in addition to time of day) may contribute to the ambiguous findings reported within this area. Based upon the results found in this study, an evening-type tested in the morning would display lower cardiac levels than if they were tested in the early afternoon. Moreover, in studies where morningness – eveningness remains an unmeasured variable, the effects of any experimental manipulation or individual differences variable within a stress paradigm may well be masked or even exaggerated. Thus increasing the likelihood of type 1 and/or type 2 errors, depending upon the nature of the study. Therefore, we would suggest that future research should endeavour to control for both time of testing and morningness – eveningness, in order to ensure the influence of these two factors can be accounted for. We also found the synergistic effects of morningness – eveningness and time of testing were restricted to HR and they did not impact upon BP. This is congruent with Nebel et al. [12], who, across two small studies, also found consistent evidence of a significant interaction between morningness –eveningness and time of testing for HR, but not for BP. Taken together, these findings suggest that HR may be more closely associated with morningness– eveningness than BP, and consequently is more likely to be influenced by this variable. However, without further research, it is difficult to provide an adequate explanation for this finding. It is also important to note that the highest HR levels exhibited in this study were observed in the evening types during the afternoon session. This finding is not altogether unsurprising given that previous research has found that evening types are more likely to exhibit behaviour patterns typical of a Type A personality [39]. Therefore, it is possible that specific aspects of the evening-type personality (e.g. perhaps being high in cynical hostility or anger) may mediate the effects of morningness– eveningness observed in this study. Future research should attempt to replicate these findings and investigate the potential underlying mechanisms. In addition, as predicted this study also found evidence for a time of day effect in self-reported anxiety. Participants reported themselves to be significantly more anxious in the morning session than in the afternoon. However, there was no evidence of an interaction between time of testing and morningess – eveningness. While this conflicts with the findings of Nebel et al. [12], it is in accordance with

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another study that showed mood to be lower at 0800 h, relative to 1400 h and 2300 h [40]. These findings can be viewed in line with the evidence that cardiovascular events such as myocardial infarction and stroke occur most frequently in the morning hours [10]. Emotional distress features prominently in cited triggers of MI [e.g. 41,42], and it is therefore possible that a heightened propensity to feel anxious in the morning could be in some way related to the mechanisms causing the higher incidence of cardiovascular events at this time. This possibility warrants a more detailed investigation in a large clinical sample. The results of this study also highlighted the importance of considering gender in stress-cardiovascular activity research. The current findings corroborate existing evidence that males possess a greater resting SBP, while females show higher resting HR [21]. Studies of gender differences in reactivity have tended to produce more inconsistent findings, however. Despite finding no gender difference in change scores, we found that male SBP reached higher levels during stress, while females displayed higher HR. This is in keeping with research that has indicated that males show greater SBP reactivity than females [e.g. 43], while females’ HR may be more reactive than males [23]. It has also been suggested that this phenomenon may be related to differences in hemodynamic processes. For example, Allen, Stoney, Owens and Matthews [44] and Girdler and Light [45] have proposed that males tend to be Fvascular reactors_, while females tend to be Fmyocardial reactors_. This difference relates to the mechanisms by which blood pressure is raised in response to stress: in women, there is evidence that it is raised mainly through cardiac output; in men, increased vascular resistance mediates the rise in blood pressure (see 44). Recent work by Veldhuijzen Van Zanten et al. [42] found that during stress, males showed more pronounced rises in hematocrit concentration, as well as a greater fall in plasma volume, relative to females. The reasons for these differences remain unclear, although it has been suggested that gender variations in hormone levels may play a part. The higher levels of estrogen in premenopausal women may result in their lower vascular resistance [46]. Clearly, the study of gender differences and cardiovascular stress reactivity will continue to be an area of great interest. Finally, it is important to consider two potential shortcomings of the current study. First, we acknowledge that the sample size utilised was relatively small, although, given the repeated measures nature of the design, it is unlikely that this may have had a substantial impact on the results. Moreover, it may have reduced the likelihood of finding clearer, significant effects. Nonetheless, it would be prudent to replicate these effects in a larger clinical and non-clinical sample. Second, we recognise that the participants were tested comparatively early in the afternoon condition (i.e., between 1– 2 pm) and that the influence of morningness– eveningness may have been more apparent if they were tested later in the day (i.e., between 2 –5 pm). This remains

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a possibility, and we would suggest that future research should ensure that a laboratory testing session is scheduled later in the afternoon to examine this issue. A further possibility would be to collect measurements over the course of the day and investigate the circadian profile of cardiovascular reactivity to stress. The circadian profile of cardiovascular activity at rest is relatively well established but little is known about how parameters of reactivity may differ with time. However, notwithstanding these limitations, the results of this study provide evidence that morningness– eveningness, in conjunction with time of day, has a significant influence on cardiovascular activity both at rest and in response to stress. These findings may have implications for cardiovascular health and for existing and future stress research.

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