The effects of prior light history on the suppression of melatonin by light in humans

: We investigated the impact of light exposure history on light sensitivity in humans, as assessed by the magnitude of the suppression of melatonin secretion by nocturnal light. The hypothesis was that following a week of increased daytime bright‐light exposure, subjects would become less sensitive to light, and that after a week of restriction to dimmer light they would become more sensitive. During the bright week, subjects (n = 12) obtained 4.3 ± 0.4 hr of bright light per day (by going outside and using light boxes indoors). During the dim week, they wore dark goggles (about 2% light transmission) when outside during daylight and spent 1.4 ± 0.9 hr per day outside. Saliva samples were obtained every 30 min for 7 hr in dim light (