Natural light exposure of young adults

Physiology&Behavior, Vol. 38, pp. 571-574. Copyright©Pergamon Press Ltd., 1986. Printed in the U.S.A.

0031-9384/86 $3.00 + .00

Natural Light Exposure of Young Adults T H O M A S J. S A V I D E S , S A M M E S S I N , CHARLES SENGER AND DANIEL F. KRIPKE 1

Department of Psychiatry, University of California, San Diego and San Diego Veterans Administration Medical Center San Diego, CA 92161 R e c e i v e d 15 M a y 1986 SAVIDES, T. J., S. MESSIN, C. SENGER AND D. F. KRIPKE. Natural light exposure of young adults. PHYSIOL BEHAV 38(4) 571-574, 1986.--Bright light has a role in natural coordination of mammalian circadian and seasonal rhythms. In humans, the light intensity must probably exceed 2000 lux to be optimal. Natural light exposures of 10 healthy adults were measured over a 24-hour period, using forehead illumination transducers connected to a portable computer. The subjects varied markedly in duration and timing of exposures to light greater than 2000 lux. On average, the subjects experienced bright light for only 90 minutes per day, less than the 3-8 hours of bright light necessary to maximally synchronize human circadian rhythms. These results suggest that natural and artificial light exposure for many Americans may be suboptimal for circadian and seasonal synchronization. Circadian rhythms

Melatonin

Illumination

Photometry

IN adaptation to the earth's daily and seasonal light/dark cycles, many organisms, including humans, have developed internally timed rhythms of physiology and behavior. In animals, the 24-hour light/dark cycle synchronizes daily rhythms, such as sleep and body temperature, while changing lengths of daylight throughout the year cue seasonal rhythms such as reproduction [2,12]. The environmental light/dark cycle also indirectly synchronizes the nocturnal secretion of melatonin from the pineal gland of mammals [16]. Pinealocyte synthesis of melatonin is stimulated by an endogenous pacemaker thought to be located in the suprachiasmatic nucleus (SCN) of the hypothalamus [10]. The circadian rhythm of this endogenous pacemaker is entrained to the environmental light/dark cycle through input from the retinohypothalamic tract, which transmits photic information from the retina to the SCN [11]. The SCN-pineal system has crucial roles both in regulation of daily behavior and in the endocrine control of reproduction [12]. Recent evidence indicates that light as bright as 15002500 lux can suppress nocturnal melatonin synthesis in humans [9]. Similar intensities can synchronize human circadian rhythms experimentally [17,18]. This suggests that high intensity light (bright daylight or very bright artificial light) might have a role in natural coordination of circadian and seasonal rhythms in humans. Indeed, analyses by Aschoff indicate that sunlight intensities have effects on human annual rhythms beyond those of daylight dimmed by cloud cover [1]. In order to study natural light exposure of human sub-

Light

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jects, our group has developed a portable photometric recording system which continuously samples the intensity of illumination reaching wearable phototransducers [13]. A small computer worn around the waist records illumination every five minutes as the subject goes about his/her usual activities at work, at home, or in recreation. In this study, we specifically examined the duration and timing of illumination exposures exceeding 2000 lux, which are capable of suppressing human nocturnal melatonin synthesis. Ten healthy adult subjects (ages 21-42) volunteered to participate in this study between August 22 and November 6, 1984 in San Diego, CA. Informed consent was obtained after the nature of the study had been explained. Recordings were commenced at varied times of day, but each subject wore the recording system continuously for 24 hours while maintaining normal daily activities. For contrast to the summer/fall subject recordings, and to determine the minimum illumination during the year, a 24hour recording of winter outdoor illumination was obtained on a clear day, February 15, 1985. The illumination transducer was directed toward a panoramic view of the northern horizon from a location about 100 feet above ground level in a suburban area. The photometric recording system consisted of a Vitalog PMS-8 computer-based monitor (Vitalog Corporation, 643 Bair Island Road, Suite 300, Redwood City, CA 94063) connected to illumination transducers at the forehead and wrist. Each transducer consisted of a photoresistor (CL9P911 photoconductive resistor), fixed resistor, and battery net-

~Requests for reprints should be addressed to Daniel F. Kripke, Department of Psychiatry (116A), VA Medical Center, 3350 La Jolla Village Drive, San Diego, CA 92161.

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work connected to the Vitalog analog-to-digital converter [6]. Blue Kodak Wratten Gelatin Filter (No. 38) and Kodak Neutral Density Attenuator Film (Density 1.10, No. 1734532) were affixed over the photocell aperture to produce a system with a maximal sensitivity at about 520 nanometers, close to the wavelengths which maximally suppress melatonin synthesis [3]. The range of sensitivity was from 10 to 50,000 lux, covering a range from dim twilight to direct sunlight. A receptive angle of almost 180 degrees was obtained, closely resembling the receptive field of retinal neurons innervating the suprachiasmatic nucleus [4]. Figure 1 illustrates forehead illumination recordings for all 10 subjects plus the outdoor winter recording for comparison. The intensity and timing of illumination each subject received varied depending on activities and sleep schedules. Subjects A, B, D, E, and I were dayshift medical researchers

recorded during their work week. These researchers worked in offices without exterior windows. Subjects F and J were part-time medical researchers who worked during the morning and spent the afternoon outdoors in recreation (beach, tennis, bike riding). Subject G attended class in the morning and played frisbee outdoors in the afternoon. Subjects C and H spent most of the day indoors at home. Figure 2 illustrates the mean daily pattern of illumination and the inter-subject variability in illuminations at different times of day. In general, illumination exposure increased after 6:00 a.m., peaking around 3:00 p.m. At no time did the geometric mean illumination for the group reach 2000 lux. There were, however, two distinct peaks when many subjects received illumination exceeding 2000 lux. The first occurred during the morning between 8:00 and 10:30 a.m., corresponding to commuting to work. All eight of the subjects who commuted to work or school experienced illumination greater than 2000 lux briefly (5-35 min) between 8:00 and 10:30 a.m. A second mean illumination peak, occurring between noon and 6:00 p.m., was created almost entirely by the three subjects who participated in the afternoon recreation outdoors. The subjects experienced illumination exceeding 2000 lux for a mean of 6%, of the 24 hours, that is, about 1.5 hours (range 1.0-23.2%) (Fig. 3). The 5 medical researchers spent only a mean of 1.8% of the 24 hours or 26 minutes in illumination exceeding 2000 lux (range 1.0-2.7%). The subjects who spent afternoons in outdoor recreation averaged 16.7% of the 24 hours, or about 4 hours, in illumination greater than 2000 lux (range 12.8-23.2%). The two subjects who spent the day at home indoors experienced an average of 2.4% of the 24 hours, or about 30 minutes in illumination greater than 2000 lux (range 2.0-2.7%). Most of this exposure occurred during the commute back and forth to the research laboratory when picking up and returning the recording instrument. These exposures contrast with the nearly 10 hours of illumination greater than 2000 lux measured outdoors during a winter day. This study illustrates the extreme variability in the duration and timing of bright light exposures greater than 2000 lux in contemporary humans. However, none of the subjects were exposed to durations of bright illumination approaching the natural photoperiod. Even outdoor illumination on a short winter day greatly exceeded human bright light exposure in the summer. Wever et al. [17,18] showed that artificial light exposures of approximately 3000-4000 lux synchronize human circadian rhythms more effectively than less intense light or auditory signals, but only if the exposures last at least 3-8 hours. Only those subjects who spent the entire afternoon outdoors experienced comparable illumination. The average amount of bright light experienced by the researchers, whose schedules might be more representative of working Americans, was only 26 minutes. People living in areas of the country more cloud-covered, polluted, metropolitan, or in more northern latitudes than San Diego in winter probably experience even less illumination. Thus, natural and artificial light exposure for many Americans may be insufficient for optimal circadian synchronization. Circadian rhythms of animals kept in constant darkness will advance or delay depending on when in the circadian cycle the animal is exposed to a brief pulse of light [14]. This observed relationship between the time in the animal's subjective day when a light pulse is given and the phase shift obtained can be plotted as a phase response curve (PRC).

N A T U R A L LIGHT EXPOSURE OF YOUNG ADULTS

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Only light pulses experienced near the subjective night (usually about 12 hours surrounding mid-dark) alter rhythms. Light pulses near the animal's subjective dawn advance rhythms, whereas light pulses near the subjective evening delay rhythms. Light pulses during the subjective day may have no effect on rhythm synchronization. An analogous PRC in humans has been hypothesized [8]. In this study, some subjects may have experienced bright light exposures only during the morning phase of the active PRC, whereas others mainly experienced bright light during the evening part of the active PRC. Thus it is possible that the subjects experienced quite different synchronizer effects upon their circadian systems, some receiving delaying stimuli and others receiving advancing stimuli. While the duration and timing of daylight exposure profoundly affects endocrine functions and behavior in some mammals, little is known concerning similar photoperiodism in humans. However, the existence of annual rhythms in hormones, such as plasma testosterone [1], and psychiatric illnesses, such as Seasonal Affective Disorder (SAD) [15], suggest humans may respond to photoperiodic mechanisms.

The improvements in depressed mood seen in some patients treated with bright light exposures indicate that bright light can have profound health benefits in discrete circumstances [5, 7, 15]. It is unknown whether brief periods of bright light can entrain human circadian rhythms. The Wever study showed that 3 to 8 hours of bright illumination were required for maximal entrainment of circadian rhythms significantly different from the entrainment caused by dim light or auditory signals [ 17,18]. No one has systematically tested the efficacy of shorter durations of bright light exposure on entrainment, and so current data are insufficient to show whether the 26 minutes of bright illumination experienced by the medical researchers in our study are sufficient for maximal entrainment. In addition, it is unclear whether morning light is more important than evening light in entraining rhythms, although this might be suspected to be true based on the hypothesized human PRC [8]. There is some unpublished evidence that pulses of light as short as 30 to 120 minutes may be sufficient to treat SAD patients. However it is unclear whether this treatment works through circadian entrainment or through some other mechanism. It cannot be assumed that the SAD effect is the same as an entrainment effect. In conclusion, this study shows that the timing and total duration of daylight exposure for modern industrial man differs markedly from the natural conditions to which our species has evolved. It is of concern that dim artificial lighting may alter contemporary people's adaptation to our natural environment. Considering known effects of bright light c a animal circadian and seasonal rhythms, we should explore the relationship of the timing and duration of daily light exposures to sleep disorders, to mood, and to endocrine responses controlling fertility, growth, and puberty. If contemporary patterns of light exposure are too weak and erratic to optimally synchronize human circadian and seasonal rhythms, correction of lighting exposures could offer exciting improvements in human adaptation. ACKNOWLEDGEMENTS S u p p o r t e d b y N I M H 2 R01 M H 3 8 8 2 2 , b y N I M H R S D A 5 K02 M H 0 0 1 1 7 (to D . F . K . ) , b y N H L B I / N R S A H L 0 7 4 9 1 - 0 5 (to T.J.S.), a n d b y the V e t e r a n s A d m i n i s t r a t i o n . W e t h a n k S a m G a b r i e l , J o h n W e b s t e r a n d the v o l u n t e e r s u b j e c t s for t h e i r a s s i s t a n c e .

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