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

Abstract

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 (<15 lux) on two consecutive nights (baseline and test night) at the end of each week. On the test night, 500 lux was presented for 3 hr in the middle of the collection period to suppress melatonin. There was significantly more suppression after the dim week compared with after the bright week (to 53 versus 41% of the baseline night values, P < 0.05). However, there were large individual differences, and the difference between the bright and dim weeks was most pronounced in seven of the 12 subjects. Possible reasons for these individual differences are discussed, including the possibility that 1 wk was not long enough to change light sensitivity in some subjects. In conclusion, this study suggests that the circadian system's sensitivity to light can be affected by a recent change in light history.

Fig. 1

Melatonin profiles from seven subjects who showed more melatonin suppression after the dim week than after the bright week (subjects C08, C09, C10, C13, C25, C26 and C28). Vertical lines indicate the period of 500-lux light exposure on the test nights. At all other times the light intensity was <15 lux. Profiles were averaged so that the 3 hr of light exposure were aligned. The clock times shown are for subjects who had the light exposure from 1:00 to 4:00. The others had the light from 0:30 to 3:30. Before group averaging, each profile was converted into percentages of its respective baseline post-value. The baseline post-value was the average melatonin level of the last four samples collected in the 3-hr interval during the baseline night, e.g. 2:30, 3:00, 3:30 and 4:00.

Fig. 2

Melatonin profiles from two subjects who showed similar suppression after the dim week compared with after the bright week (subjects C11 and C14). For more details see Fig. 1.

Fig. 3

Melatonin profiles from two subjects who had delayed profiles (subjects C19 and C21). For more details see Fig. 1.

Fig. 4

Melatonin profiles from subject C27 who had a bimodal pattern. For more details see Fig. 1.