Bright Light Resets the Human Circadian Pacemaker Independent of the Timing of the Sleep-Wake Cycle

Abstract

Human circadian rhythms were once thought to be insensitive to light, with synchronization to the 24-hour day accomplished either through social contacts or the sleep-wake schedule. Yet the demonstration of an intensity-dependent neuroendocrine response to bright light has led to renewed consideration of light as a possible synchronizer of the human circadian pacemaker. In a laboratory study, the output of the circadian pacemaker of an elderly woman was monitored before and after exposure to 4 hours of bright light for seven consecutive evenings, and before and after a control study in ordinary room light while her sleep-wake schedule and social contacts remained unchanged. The exposure to bright light in the evening induced a 6-hour delay shift of her circadian pacemaker, as indicated by recordings of body temperature and cortisol secretion. The unexpected magnitude, rapidity, and stability of the shift challenge existing concepts regarding circadian phase-resetting capacity in man and suggest that exposure to bright light can indeed reset the human circadian pacemaker, which controls daily variations in physiologic, behavioral, and cognitive function.

What Keeps Us Awake?—the Role of Clocks and Hourglasses, Light, and Melatonin

“…This blue-shift response was similarly observed in a study that compared a 2-h evening exposure to mono chromatic light of two different wavelengths (460 and 550 nm) at very low intensities, whereby subjects were more alert during the 460-nm than the 550-nm light (Cajochen et al, 2005). These findings corroborate to a wide range of non-visual light responses in humans, such as melatonin sup pression (Lewy et al, 1980;Zeitzer et al, 2000b), circadian phase shifting (Czeisler et al, 1986), nocturnal decline in EEG SWA (Cajochen et al, 1992;Münch et al, 2006), and circadian gene expression (PER2) in oral mucosa . Common to these responses is that they are all more sensitive to short wavelength light.…”

Enhanced performance in elderly subjects following bright light treatment of sleep maintenance insomnia

“…Reed Performance Assessment Battery (Thorne et al 1985), Immediately following the period of home treatment, subjects is sensitive to effects of bright light exposure during sleep returned to the laboratory for a 4-d protocol that was identical deprivation (Campbell and Dawson 1990;Badia et al 1991; to the pre-treatment protocol, with the exception that all Murphy et al 1995), as well as to the circadian rhythm of subjects were exposed to light during the first two nights in performance (Gillooly 1990;Badia et al 1991;Murphy et al the laboratory (i.e. the adaptation nights).…”

Testing the Hypothesis of a Circadian Phase Disturbance Underlying Depressive Mood in Nonseasonal Depression

“…To induce a clear shift in circadian rhythms, 3 days of bright light exposure at the appropriate time of day, while preventing light exposure during the other sensitive parts of the phase response curve, are sufficient in healthy subjects (Czeisler et al, 1986;Dijk et al, 1987). Light therapy induces quick mood changes in seasonal affective disorders in about the same time span (3-5 days) (Meesters et al, 1995), although longer treatment durations have been reported to result in further improvement (Bauer et al, 1994;Labbate et al, 1995).…”

“…Phase positions were assessed at the end of each condition only. According to empirical (Czeisler et al, 1986;Minors et al, 1994) and simulated data (Daan and Beersma, 1992), the washout period of 11 to 12 days in between ML and EL was considered to be long enough to recapture baseline phase positions at the start of the next condition. Not 1 subject showed an unexpected phase shift of DLMO after bright light exposure.…”

“…This can be achieved by exposing subjects to light at sensitive phases of the circadian system, that is, times at which it may be expected to produce phase shifts. In healthy subjects, 2 or 3 days of bright morning or evening light produced substantial shifts (Czeisler et al, 1986;Dijk et al, 1987) and amplitude changes (Jewett et al, 1991(Jewett et al, , 1994 in circadian rhythms. The second possibility to induce phase-angle differences between circadian rhythms and sleep is to force a sleep shift while keeping the other clock-related rhythms unchanged.…”