Nonentrained circadian rhythms of melatonin in submariners scheduled to an18-hour day

The human circadian timing system has previously been shown to free run wit h a period slightly longer than 24 h in subjects living in the laboratory u nder conditions of forced desynchrony. In forced desynchrony, subjects are shielded from bright light and periodic time cues and are required to live on a day length outside the range of circadian entrainment. The work schedu le used for most personnel aboard American submarines is 6 h on duty altern ating with 12 h off duty. This imposed 18-h cycle is too short for human ci rcadian synchronization, especially given that there is no bright-light exp osure aboard submarines. However, crew members are exposed to 24-h stimuli that could mediate synchronization, such as clocks and social contacts with personnel who are living on a 24-h schedule. The authors investigated circ adian rhythms of salivary melatonin in 20 crew members during a prolonged v oyage on a Trident nuclear submarine. The authors found that in crew member s living on the 18-h duty cycle, the endogenous rhythm of melatonin showed an average period of 24.35 h (n = 12, SD = 0.18 h). These data indicate tha t social contacts and knowledge of clock time are insufficient for entrainm ent to a 24-h period in personnel living by an 18-h rest-activity cycle abo ard a submarine.