title: Nonentrained circadian rhythms of melatonin in submariners scheduled to an 18-hour day.
abstract: The human circadian timing system has previously been shown to free run with a period slightly longer than 24 h in subjects living in the laboratory under 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 schedule used for most personnel aboard American submarines is 6 h on duty alternating with 12 h off duty. This imposed 18-h cycle is too short for human circadian synchronization, especially given that there is no bright-light exposure 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 circadian rhythms of salivary melatonin in 20 crew members during a prolonged voyage on a Trident nuclear submarine. The authors found that in crew members 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 that social contacts and knowledge of clock time are insufficient for entrainment to a 24-h period in personnel living by an 18-h rest-activity cycle aboard a submarine.
authors.affiliation: Naval Health Research Center, San Diego, CA 92186-5122, USA.
authors.forename: T L
authors.initials: TL
authors.lastname: Kelly
authors.forename: D F
authors.initials: DF
authors.lastname: Neri
authors.forename: J T
authors.initials: JT
authors.lastname: Grill
authors.forename: D
authors.initials: D
authors.lastname: Ryman
authors.forename: P D
authors.initials: PD
authors.lastname: Hunt
authors.forename: D J
authors.initials: DJ
authors.lastname: Dijk
authors.forename: T L
authors.initials: TL
authors.lastname: Shanahan
authors.forename: C A
authors.initials: CA
authors.lastname: Czeisler
mesh_terms: D000328:Adult; D002940:Circadian Rhythm / Q000502:physiology*; D006801:Humans; D008297:Male; D008550:Melatonin / Q000378:metabolism*; D008889:Military Personnel; D017440:Photoperiod; D012463:Saliva / Q000378:metabolism
chemical_list: D008550:Melatonin