Acute effects of bright light and caffeine on nighttime melatonin and temperature levels in women taking and not taking oral contraceptives

Caffeine and bright light effects on nighttime melatonin and temperature le vels in women were tested during the luteal phase of the menstrual cycle (n =30) or the pseudo luteal phase for oral contraceptive users (n=33,). Parti cipants were randomly assigned to receive either bright (5000 lux) or dim r oom light (<88 lux) between 20:00 and 08:00 h under a modified constant rou tine protocol. Half the subjects in each lighting condition were administer ed either caffeine (100 mg) or placebo in a double-blind manner at 20:00, 2 3:00, 02:00 and 05:00 h. Results showed that the combination of bright ligh t and caffeine enhanced nighttime temperature levels to a greater extent th an did either caffeine or bright light alone. Both of the latter groups had higher temperature levels relative to the dim light placebo condition and the two groups did not differ. Temperature levels in the bright light caffe ine condition were maintained at near peak circadian levels the entire nigh t in the luteal and pseudo luteal phase. Melatonin levels were reduced thro ughout the duration of bright light exposure for all women. Caffeine reduce d the onset of melatonin levels for women in the luteal phase, but it had l ittle effect on melatonin levels for oral contraceptive users. The results for women in the luteal phase of the menstrual cycle are consistent with ou r previous findings in men. The results also suggest that oral contraceptiv es may alter the effects of caffeine on nighttime melatonin levels. (C) 200 0 Elsevier Science B.V. All rights reserved.