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.