MELATONIN RHYTHM IS NOT SHIFTED BY LIGHTS THAT SUPPRESS NOCTURNAL MELATONIN IN HUMANS UNDER ENTRAINMENT

Effects of a single light exposure on the circadian rhythm in plasma m elatonin were examined in young males to obtain the threshold of light intensity for suppressing the nocturnal melatonin level on the one ha nd and to understand the relationship between the light-induced phase shift of melatonin rhythm and the melatonin suppression on the other h and. Eight subjects spent 3 days in an experimental living facility wh ere light intensity was set below 200 lx and were exposed to light for 3 h in the early morning on the 2nd day. The same procedure was repea ted five times in each subject with an interval of at least 3 wk, and one of five light intensities was tested in each trial. As a result, n octurnal melatonin level was not suppressed by light of 200 lx but sig nificantly suppressed by light of intensity greater than or equal to 5 00 lx. On the other hand, the circadian melatonin rhythm was not shift ed by any light intensity up to 10,000 lx. It is concluded that the th reshold of light intensity for suppressing the melatonin level is loca ted between 200 and 500 lx in young Japanese males, and the threshold for phase shifting the circadian melatonin rhythm was much greater tha n that for suppressing the nocturnal melatonin level in humans under e ntrained conditions.