The effect of low light intensity on the maintenance of circadian synchrony in human subjects

Experiments were conducted on six healthy male subjects aged 20-23 yr and exposed for 21 days in a confined regulated environment to 16L:8D light:dark cycle with a view toward determining whether the light environment of 16L:8D at the relatively low light intensity of 15 ft.c. is adequate for the maintenance of circadian synchrony in man. The light intensity was 100 ft.c. during the first seven days, reduced to 15 ft.c. during the next seven days, and increased again to 100 ft.c. during the last seven days. Rectal temperature (RT) and heart rate (HR) were recorded throughout the three phases. In the 100 ft.c. regime, the RT and HR rhythms remained stable and circadian throughout. It is shown that 15 ft.c. light intensity is at or below threshold for maintaining circadian synchrony of human physiologic rhythms marked by instability and internal desynchronization with degradation of performance and well-being.