THE EFFECT ON BODY-TEMPERATURE AND MELATONIN OF A 39-H CONSTANT ROUTINE WITH 2 DIFFERENT LIGHT LEVELS AT NIGHTTIME

Eight healthy subjects were studied during 39-h spans (from 07:00 on o ne day until 22:00 the second) in which they remained awake,During one experiment, subjects were exposed to 100 lux of light between 18:00 a nd 8:00, and during a second experiment, they were exposed to 1000 lux during the same time span. Throughout the daytime period, they were e xposed to normal daylight (1500 lux or more). The nighttime 1000-lux l ight treatment suppressed the melatonin metabolite aMT6s, while the 10 0 lux treatment did not. On the treatment day, the 1000 lux, in compar ison to the 100 lux, light treatment resulted in both an elevated temp erature minimum and a delay in its clock-time occurrence overnight. No real circadian phase shift in the temperature, urinary melatonin, or cortisol rhythms was detected after light treatment. This study confir med that nocturnal exposure to lower light intensities is capable of m odifying circadian variables more than previously estimated. The immed iate effects of all-night light treatment are essentially not differen t from those of evening light. This may be important if bright light i s used to improve alertness of night workers. Whether subsequent dayti me alertness and sleep recovery are affected by the protocol used in o ur study remains to be determined.