Effect of 18-h watch schedules on circadian cycles of physiological functions during submarine patrols.

Circadian rhythms of body temperatures, pulse rate, and respiration rate were measured in 11 subjects every 4 h during certain periods on two submarine patrols. Data on systolic and diastolic blood pressure were also obtained on five crew members during the first period. All the subjects of the first patrol were on an 18-h watch schedule (6 h on, 12 h off). During the second patrol, three subjects were on an 18-h watch schedule and three were on a 24-h watch schedule. Cosinor analysis for positive (P less than 0.05) detection of rhythm demonstrated that all subjects on the 18-h watch schedule developed 18-h cycles of body temperature, pulse rate, respiration rate, and systolic and diastolic blood pressure, which were then superimposed on the persisting 24-h cycles of the same function. The three subjects on a 24-h watch schedule did not show the 18-h cycles. Moreover, additional 12-, 36-, and 48-h cycles (harmonics and subharmonics of 24-h cycles) were found in all subjects on both patrols, attesting to the disintegration of circadian cycles under these conditions. Average sleep time tended to decrease toward the end of the patrol.