EFFECTS OF BRIGHT LIGHT ON CIRCADIAN PATTERNS OF CYCLIC ADENOSINE-MONOPHOSPHATE, MELATONIN AND CORTISOL IN HEALTHY-SUBJECTS

Bright light is known as a strong zeitgeber on human circadian rhythms and influences several endocrine and neuroendocrine functions. In the present study we examined the influence of. a 3-h bright light stimul us, given at different times during the day (morning or evening), on c ircadian patterns of cyclic adenosine monophosphate (cAMP), melatonin and cortisol. Two groups of synchronized healthy volunteers (lights on : 05.00-23.00 h) were exposed to bright light (2500 lux) for 3 h over 6 days either in the morning (05.00-08.00 h) or in the evening (18.00- 21.00 h). The results showed a significant phase advance in the circad ian rhythms of melatonin and cortisol when bright light was given in t he morning but not when given in the evening. Rhythm in plasma cAMP ba sically was not affected by either light treatment.