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Adaptation of Human Pineal Melatonin Suppression by Recent Photic History

Division of Sleep Medicine, Department of Medicine, Brigham & Women’s Hospital, Harvard Medical School, Boston, Massachusetts 02115

Address all correspondence and requests for reprints to: Charles A. Czeisler, Ph.D., M.D., Division of Sleep Medicine, Brigham & Women’s Hospital, Harvard Medical School, 221 Longwood Suite 438, Boston, Massachusetts 02115. E-mail: caczeisler{at}hms.harvard.edu.

	Abstract

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The human circadian pacemaker controls the timing of the release of the pineal hormone melatonin, which promotes sleep, decreases body temperature, and diminishes cognitive performance. Abnormal melatonin secretion has been observed in psychiatric and circadian disorders. Although melatonin secretion is directly suppressed by exposure to light in a nonlinear intensity-dependent fashion, little research has focused on the effect of prior photic history on this response. We examined eight subjects in controlled laboratory conditions using a within-subjects design. Baseline melatonin secretion was monitored under constant routine conditions and compared with two additional constant routines with a fixed light stimulus for 6.5 h of 200 lux (50 µW/cm²) after approximately 3 d of photic exposure during the subjective day of either about 200 lux (50 µW/cm²) or about 0.5 lux (0.15 µW/cm²). We found a significant increase in melatonin suppression during the stimulus after a prior photic history of approximately 0.5 lux compared with approximately 200 lux, revealing that humans exhibit adaptation of circadian photoreception. Such adaptation indicates that translation of a photic stimulus into drive on the human circadian pacemaker involves more complex temporal dynamics than previously recognized. Further elucidation of these properties could prove useful in potentiating light therapies for circadian and affective disorders.

	Introduction

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MELATONIN IS SECRETED during the subjective night in response to sympathetic drive originating from the human circadian pacemaker, located in the suprachiasmatic nucleus of the hypothalamus (1). This secretion can be acutely suppressed by light. This suppression response bears a nonlinear relationship to the intensity of the light stimulus (2) and is more sensitive to lower light intensities than previously hypothesized (3). Exogenously administered melatonin can also entrain circadian rhythms (4). Light exposure resynchronizes, or resets, the hypothalamic circadian pacemaker, phase-shifting hormone rhythms, core body temperature, and timing of sleep-wake cycles (1, 5) and has been implicated in a wide variety of sleep and circadian disorders (1).

Little research has focused on the influence of prior photic history on the suppressive effect of light on pineal melatonin secretion. The resetting response of the circadian pacemaker can be reduced by a preceding nonsaturating stimulus in animals (6), and phase shifting in mammalian models has been shown to be maximal after prolonged exposure to complete darkness before a stimulus (7, 8). Wide variations in the sensitivity of the resetting response have been reported in humans (2, 9, 10, 11), but most investigators did not account for the potential effect of light exposure before the resetting stimulus. These observations suggest that the relationship between light intensity and melatonin suppression may not be static as implied by previous reports (2, 9, 10, 11), but instead may exhibit modulation due to prior photic history. We aimed to evaluate adaptation in the melatonin suppression response of the human circadian pacemaker with regard to prior light exposure in controlled laboratory conditions.

	Subjects and Methods

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Ten subjects were admitted to the inpatient portion of the study. Two subjects withdrew from the study before completion and were excluded from further analysis. Eight healthy volunteers (ages 18–29 yr, five males and three females) completed the study. All subjects were free of any medical condition, sleep complaints, or medications. Screening included medical examination, standard blood and urine chemistry analysis, and psychological testing. Participants were required to refrain from any foreign substance throughout the study, which was confirmed via periodic toxicological screen. Three weeks before inpatient study, a strict 16-h wake, 8-h sleep schedule was maintained and was verified by daily call-ins to a time-stamped voicemail. One week before inpatient entry, each subject’s schedule was verified to be within 15 min of the reported sleep-wake times by wrist actigraphy. Before the inpatient protocol, written informed consent was obtained in accordance with the Institutional Review Board of the Brigham & Women’s Hospital, and all procedures were carried out in accordance with the Declaration of Helsinki.

Subjects were studied individually in a specialized environment of uniform light intensity free from time cues. Light intensity and sleep-wake schedule were maintained constant across subjects. Nutritional intake was calculated as an isocaloric diet based on individual metabolic requirements. Participants refrained from napping or exercising during the waking portions of the study and did not leave the suite for the duration of the protocol. Continuous blood sampling was obtained via intravenous catheter. All lighting throughout the study was provided by ceiling-mounted fluorescent lamps (F96T12/41U and F32T8/ADV841; Phillips, Eindhoven, Netherlands) shielded with clear polycarbonate filters to remove 99.9% of UV spectrum light. Light measurements were monitored during the study using an IL-1400 photometer (International Light, Newburyport, MA).

The inpatient portion of the protocol was 14 d in three separate conditions (Fig. 1). Each condition began with 2 baseline days in a fixed light intensity, either approximately 200 lux (50 µW/cm²) or approximately 0.5 lux (0.15 µW/cm²). All subjects underwent melatonin assessment in constant routines that lasted 40 h, during which time they maintained a fixed semirecumbent posture, were kept awake, and remained in bed. In this constant routine, each subject received a 6.5-h light exposure during the subjective night. The light exposure in each condition was timed to begin at 14 h 55 min after habitual wake time (slightly after dim light melatonin onset) and ended at 21 h 25 min after habitual wake time. During the light exposure, the subject remained semirecumbent in the bed but was asked to fixate on a point on the opposite wall. Subjects alternated between a fixed gaze for 6 min and looking freely around the room for 6 min for the 6.5-h duration. Light measurements were taken at the forehead after every fixed gaze. The first light exposure was used as a control assessment in dim lighting, and the two experimental exposures were identical 200-lux exposures. After constant routine conditions, subjects received 8 h of sleep, and at the end of the protocol, they were discharged at their convenience. Four subjects completed each of the two crossover conditions.

View larger version (21K): [in this window] [in a new window]   FIG. 1. The protocol is presented as a standard double raster plot, with consecutive days plotted side by side. The order of d 6–9 and 10–13 was reversed for half of the subjects. Black shaded bars represent sleep episodes (0 lux). Open bars represent an ambient light level of approximately 200 lux at the angle of gaze. Each constant routine (CR) lasted 40 h. Sun icons represent light exposure periods of 6.5 h each centered at 18 h 10 min after habitual wake time. After 3 baseline days, subjects underwent an initial CR and light exposure entirely at approximately 0.5 lux for assessment of baseline melatonin secretion. Each subject then underwent 2 baseline days of either approximately 0.5 or 200 lux, after which they again underwent a CR at the same light level with a 200-lux (angle of gaze) light exposure, to examine differences in the melatonin profile during the 200-lux light exposure between the two lighting histories as well as before and after the light exposure.

	Results

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The two lighting history conditions (200 and 0.5 lux) showed significant differences in melatonin suppression during identical stimulus intervals, with a mean suppression of 71.2% (±7.1%) in the approximately 200-lux prior light history condition vs. a mean suppression of 85.7% (±6.5%) in the approximately 0.5-lux prior light history condition. Statistical analysis (one-tailed for paired samples) yielded a significant difference in suppression between the two conditions (P = 0.0120). The difference between the two conditions was highlighted during the final 3 h of the light exposure, at which time the mean melatonin suppression was 26.1% lower in the approximately 200-lux prior light history condition than it was in the approximately 0.5-lux prior light history condition. There was no significant correlation between computed melatonin peak amplitude and suppression during the light exposure episodes.

During the baseline constant routine (Fig. 2A), subjects exhibited the usual nocturnal melatonin secretory episode observed under dim light conditions (0.5 lux). Intraindividual variability in the size of the nocturnal melatonin peak over the course of the entire study was minimal. During the approximately 200-lux light exposure preceded by the approximately 200-lux lighting condition (Fig. 2B), the melatonin profile was diminished in amplitude and delayed in onset while maintaining its usual shape, consisting of a single continuous episode of melatonin secretion. In contrast, the melatonin suppression seen in the approximately 200-lux light exposure preceded by the approximately 0.5-lux lighting condition (Fig. 2C) was much more marked, with melatonin secretion either completely suppressed or held at a low level until completion of the light stimulus, at which point all subjects resumed melatonin secretion at levels similar to baseline. Dim light melatonin offset, defined as the time at which melatonin secretion dropped less than 25% of the estimated peak plasma level, was significantly later in the approximately 0.5-lux background condition (P = 0.0385, one-tailed). In the baseline constant routine, offset occurred at a mean of 1.4 h (±0.3 h) after habitual wake time, and in the approximately 200-lux prior light history condition, offset occurred at a mean of 0.3 h (±0.6 h), but in the approximately 0.5-lux prior light history condition, offset was delayed until a mean of 2.1 h (±0.7 h).

View larger version (38K): [in this window] [in a new window]   FIG. 2. Plasma melatonin secretion profiles for each of the eight subjects across the three conditions in the study. Profiles are shown for the entire 40-h constant routine (CR), with the 6.5-h light exposure (LE) demarcated on each. Shading indicates dim (0.5 lux) lighting conditions. Plasma melatonin secretion is reported in picomoles per liter. A, Baseline plasma melatonin profiles in dim light (0.5 lux). B, Plasma melatonin profiles during the approximately 200-lux CR with a 200-lux LE. Note the diminished amplitude in most subjects and the delayed melatonin secretion onset. C, Plasma melatonin profiles during the approximately 0.5-lux CR with a 200-lux LE. Note the markedly depressed melatonin amplitude during the light exposure followed by resumed melatonin secretion and the delay in melatonin onset in most subjects.

	Discussion

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Furthermore, our results demonstrate that significant melatonin suppression can be achieved at relatively low light levels, as evidenced by continual reduction in amplitude of the melatonin profile in the approximately 200-lux prior light history condition, which is consistent with previous studies (2, 12). However, there was greater melatonin suppression during the light exposure that followed the approximately 0.5-lux background condition. These data suggest that there is some adaptation of light-induced melatonin suppression between the two conditions, which is consistent with prior studies in animal models. Although melatonin levels during the first 3.5 h of light exposure appeared to be equally suppressed in both conditions, the result may have been confounded by the approximately 200-lux prior light history condition inducing a phase delay in the melatonin onset before the stimulus. The significant difference in the final 3 h of light exposure underscores the difference in melatonin secretion between the two conditions. Additionally, we observed one individual who was totally suppressed by approximately 200 lux of light and one individual who showed little suppression by approximately 200 lux of light, which highlights the range of variation in absolute photic sensitivity among healthy individuals.

Melatonin has been shown to influence the suprachiasmatic nucleus and ultimately feed back on the human circadian pacemaker, and there is evidence that mechanisms of melatonin suppression and resetting effects on the human circadian pacemaker follow similar neural pathways (13). Unfortunately, no definitive conclusions can be reached from this protocol regarding the phase-shifting effects of the light exposure stimulus because it is difficult to assess during the stimulus whether shifts in the melatonin profile represent actual phase shifting of the circadian pacemaker or an acute light-induced melatonin suppression response (14). Further substantiation of such a phenomenon would require an analysis of circadian markers not confounded by lighting conditions in an experiment optimized for observing phase shifting.

One striking feature of these data is the low light levels at which melatonin suppression occurred, in contrast to prior research. Several prior studies have shown that low-level light exposures around 200 lux have been insufficient to suppress melatonin, but prior light history was not rigorously controlled before testing (10, 11, 15, 16). Our results agree with the nonlinear intensity-suppression relationship established by Zeitzer et al. (2), which estimated greater than 50% suppression at approximately 100 lux, and suggest that the background light intensities used in many prior studies may have desensitized the circadian photoreceptive system and thereby elevated the apparent threshold of melatonin suppression. Our findings indicate that prior light exposure, even at relatively modest levels, should be both considered and controlled in studies involving examination of melatonin suppression.

The modulation of the suppressive effects of light on melatonin secretion by prior light history may require a revision of our current understanding of human pineal melatonin secretion and the human circadian pacemaker. Kronauer et al. (17) have proposed a simplified model in which optic input drives the circadian oscillator by a dynamic balance of ready and used elements. This model does account for some of the findings of this study because the generation of a sizable pool of used elements during prior photic exposure would diminish the stimulus drive from a subsequent light stimulus. However, the magnitude of the difference observed in this study, combined with the gradual secretion of melatonin during the approximately 200-lux prior light history condition (compared with the total suppression of melatonin secretion until the removal of the stimulus in the 0.5-lux prior light history condition) suggests that the regeneration rate for such elements is either temporally much longer than originally suggested or there is a previously unrecognized adaptation of the system to tonic light exposure.

In this protocol, each condition had a controlled light exposure for 32 h during the prior 2 d (8-h sleep episodes were at <0.2 lux) as well as the preceding 14 h 55 min before the stimulus. Although our findings indicate that a controlled photic history over 63 h before a light stimulus is sufficient to change the suppression effect of the subsequent light stimulus, we can neither determine from this study the number of hours of prior light exposure that would be necessary to induce such an adaptive response nor determine whether differences in sensitivity between subjects might be due to adaptive responses that take place over a longer time interval. Further investigation is required to determine the time course over which photic history induces adaptation of circadian photoreception. Further studies will also be required to determine whether this adaptation response is mediated via the classic photoreceptors, which are known to exhibit adaptive responses and which have input to the circadian system (18), or via the intrinsically photosensitive retinal ganglion cells, which provide specialized input to the circadian system (19).

The implications of these results may provide insight into future treatments for circadian rhythm sleep disorders. Melatonin sensitivity to suppression is increased in bipolar disorder and possibly seasonal affective disorder (20). Recommendations for light therapy for affective and circadian disorders are necessarily broad due to individual variations in response to treatment protocols. Our results may provide novel approaches to the therapy for these disorders because attention to the entire photic environment of an individual may enhance the efficacy of such light treatments. Therefore, investigation of the underlying mechanisms of this adaptation response of the human circadian pacemaker will be critical to the effective use of this phenomenon in clinical treatment.

	Acknowledgments

 
We thank each of the patients as well as the staff of the Brigham & Women’s General Clinical Research Center for their assistance in carrying out the inpatient protocol. Special thanks to K. C. Malvey and N. McCarthy for their help in recruiting subjects, as well as J. Gooley, R. E. Kronauer, and S. Lockley for scientific discussion.

	Footnotes

 
This work was supported in part by National Institutes of Mental Health Grant 5R01-MH45130 and National Aeronautics and Space Administration Cooperative Agreement NCC9-58 with the National Space and Biomedical Research Institute and was carried out in a General Clinical Research Center supported by National Institutes of Health Grant MO1-RR02635. K.S. also received support from the Harvard-Massachusetts Institute of Technology Division of Health Sciences and Technology. K.S. and M.S. are funded in part as research fellows on National Institutes of Health General Clinical Research Center Grant MO1-RR02635.

Received December 10, 2003.

Accepted March 29, 2004.

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