Light is the main environmental cue used by the biological clock to synchronise endogenous circadian rhythms with the external day-night cycle. Therefore, various changes in circadian physiology can be expected to happen when light exposure is modified by the work schedule or by the work environment. Results from two field studies are presented. They both used ambulatory light recordings to relate the 24- h pattern of light-dark exposure with circadian entrainment. In the first study, light exposure was measured in 30 permanent night nurses to determine if specific light/dark profiles could be associated with a better circadian adaptation. Circadian adaptation was defined as a significant shift in the timing of the episode of melatonin secretion into the daytime. Light exposure was continuously recorded with ambulatory wrist monitors for 56 hours, including 3 consecutive nights of work. Subjects were then admitted to the laboratory for 24 hours to measure their circadian rhythm of melatonin secretion. Five nurses showed a circadian adaptation by phase delay ("delayed subjects") and three showed a circadian adaptation by phase advance ("advanced subjects"). The other 22 subjects had a timing of melatonin secretion typical of day-oriented people ("non-shifters"). There was no significant difference between the 3 groups for total light exposure or for bright light exposure in the morning when travelling home. However, the 24-h profiles of light exposure were very distinctive.
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