Institute of Chronobiology, Department of Psychiatry, New York Hospital-Cornell Medical Center, White Plains, New York

It is generally acknowledged that changes in the sleep/waking system accompany the aging process. Such changes are expressed most clearly in the form of fragmented, shallow nocturnal sleep, frequent, early morning awakenings and increased daytime sleepiness. The extent to which such sleep alterations may be the consequence of changes in the circadian timing system remains unclear. However, the features that characterize sleep maintenance disturbance in the elderly are consistent with circadian rhythms involvement and growing evidence supports such a notion.

We recently reported that the circadian temperature rhythms of older subjects are phase-advanced relative to young subjects and that usual sleep times, as well as subjective sleep quality, were correlated with the degree of phase advance (1). We have also reported that healthy older subjects are exposed to remarkable little light of sufficient intensity to entrain the human circadian system -- as little as 45 minutes per 24 hours (2). Based on these findings, and on the demonstrated phase-shifting effects of timed exposure to bright light, we have been attempting to readjust the phase relationship between sleep and body core temperature by exposing older, sleep-disturbed subjects to artificial bright light. We hypothesized that such phase readjustment would result in more consolidated nocturnal sleep and reduced daytime sleepiness.

METHODS: Subjects were electrographically recorded for 4 consecutive nights prior to treatment (2 Adaptation, 2 Baseline). Throughout the days following the last 2 nights in the lab, daytime sleepiness was assessed (Maintenance of Wakefulness Test), as was cognitive and psychomotor performance. Body core temperature was also continuously recorded during each subject's stay in the lab. Treatment was then administered for 7 to 10 consecutive days while subjects lived at home and continued their usual daily activities. Active treatment consisted of sitting before a bank of bright white lights (4000 to 5000 Lux) each day for two hours at a designated time, based on temperature data obtained during Baseline recordings. The control condition was identical except that subjects were exposed to dim red illumination (< 50 Lux). Following treatment, subjects again reported to the lab for 4 nights and 2 days, to repeat the pre-treatment protocol. This is an ongoing study and the finding reported here are based on the data of 6 subjects in the active condition (mean age: 68.4) and 4 subjects (mean age: 71.3) in the control condition.

RESULTS: In response to bright light exposure, subjects exhibited an average phase shift in body core temperature of about 2 hours. There was no significant phase shift in body core temperature associated with timed exposure to the dim red light. Because bedtimes and wakeup times were only slightly delayed from baseline to post-treatment, the location of the temperature minimum relative to the sleep period was altered substantially in the bright light group. Prior to treatment, the fitted minimum occurred an average of 23 minutes after the midpoint of sleep. In contrast, following timed exposure to bright light, the minimum occurred, on average, 112 minutes after the midpoint of sleep.

Significant improvements in sleep quality accompanied the circadian rhythms adjustment in the bright light group. Because the primary complaint in this sample was an inability to maintain, rather than initiate sleep, perhaps the most important and revealing variable examined was wake time after sleep onset (WASO).

On the Baseline nights, subjects in the bright light group showed an average of 91.2 minutes WASO (SD = 36.6 min). On post-treatment evaluation, WASO declined to 35.4 minutes (SD = 22.8 min) (p = .003). As a result of the reduced time awake, sleep efficiency showed a significant increase from Baseline to Post-Treatment (79.8% to 91.9%: p,.001). A further indication of improved sleep continuity was reflected in the significant reduction in stage changes following bright light exposure (p < .03). Wakefulness within sleep was replaced not only by Stage 2 sleep, but also by substantial increases in slow wave sleep (visually score Stages 3 and 4) and by REM sleep, as shown in the Figure above.

DISCUSSION: These preliminary findings indicate that timed exposure to bright light is highly effective in alleviating age-related sleep maintenance disturbance. In addition to Stage 2, both SWS and REMS percentages increased following treatment, probably because increased sleep continuity "permitted" these sleep states to evolve with fewer interruptions. Since improvement in sleep was associated with changes in the circadian timing system (as reflected by a phase shift in temperature) these data further suggest that the bright light treatment acts directly on that system, perhaps by correcting the phase relationship between body core temperature and the timing of sleep.

^*For Citation: Sleep Research (1991)20:448