Dark Skies Northwest Regional Meeting - December 2nd, 2000

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The Effects of Light on Circadian Rhythms, Sleep and Mood


David Avery, M.D. Professor,
Psychiatry and Behavioral Sciences,
University of Washington School of Medicine.


The light-dark cycle is the main synchronizer of circadian rhythms. Light stimulates the retinas, which have direct neuronal pathways to the suprachiasmatic nuclei (SCN) of the hypothalamus. The SCN appears the be the master "body clock" which coordinates multiple circadian oscillators including circadian variables such as sleep, temperature, melatonin, cortisol, and thyroid stimulating hormone (TSH).

These physiologic variables rise and fall with a period about 24 hours. Melatonin, for example, increases in the evening, plateaus during the night, and decreases in the morning. Cortisol rapidly increases in the early morning and gradually decreases during the day. Although external factors may influence the levels of these variables, these rhythms are endogenous. For example, although core body temperature may be influenced by external variables such as exercise and sleep, the temperature has a clear circadian rhythm even in subjects who are at bed rest and sleep deprived for over 24 hours. The body temperature rhythm is the most commonly studied circadian variables; often the time of the lowest point in the temperature rhythm is defined as a "hand" of the body clock. Circadian rhythms have a marked impact on sleep. Shifts in the timing of circadian rhythms can cause initial insomnia, early morning awakening, or excessive sleep.

Circadian rhythms are usually coupled and have a consistent phase relationship. For example, temperature and sleep are coupled, with sleep occurring during the low temperature trough. However, under some circumstances these rhythms may be become uncoupled. In a time-cue-free environment (no clocks, no windows), the temperature and sleep rhythms may have different periods; for example, a temperature rhythm period of 25 hours and a sleep rhythm period of 35 hours, resulting in internal desynchronization between the temperature and sleep rhythms. In such studies, most subjects have a temperature rhythm period of greater than 24 hours.

. The strength of the phase-shifting effect is dependent not only on the timing of light, but also the intensity of light. The intensity of light, illuminance, is measured in lux, which is the metric equivalent of the foot-candle. On a sunny day, the illuminance is about 50,000 - 100,000 lux; on a cloudy day, 1,000 - 10,000 lux; in a "bright well lit" office, 400 to 600 lux; in most homes, 100-300 lux; in moon light; 0.2 lux; in starlight, .001 lux. Note that even a cloudy day is much brighter than being indoors.

Initially, it was thought that only bright light was effective in shifting circadian rhythms. However, recent data indicate that even low levels of illuminance, such as 100 lux, can shift rhythms, although not as strongly as higher levels of illuminance.

Jet lag, shift work, and winter depression are examples of circadian rhythm problems that have behavioral consequences. These disorders and other potential behavioral consequences of low erratic light signals will be discussed.