This study asked how night duties might influence the circadian rhythms of healthy men working onboard a naval ship. Sixty naval personnel 20 to 30 years of age participated. Twenty who worked a normal daytime schedule from 9 AM to 6 PM constituted a control group, and the other 40 worked a night shift. Salivary levels of melatonin and cortisol were estimated from 2-hour samplings within two 24-hour periods.Typically melatonin levels began increasing just before lights-out, and 52 percent of the officers retained a normal profile during an abrupt shift in their work/rest schedule. Another 12 percent had a right phase shift in melatonin rhythm after night work, 19 percent had abnormal peak or trough levels, and 17 percent had significant reductions in melatonin throughout the sampling period. Salivary cortisol levels increased on awakening in the morning, declined during the day, and fell further to a low level just before nighttime sleep in control subjects. Erratic peaks and troughs were observed; meals and waking usually coincided with increased cortisol. Increased levels also were found in those working on radio watch, in the engine room, or on live firing exercises.Changes in work shift may lead to adverse changes in circadian rhythms for melatonin and cortisol, but there is considerable individual variation on how these rhythms respond to an abrupt phase shift in the work/rest schedule. A better understanding of these effects might suggest ways of helping military personnel to better cope with shift work. For instance, melatonin may effectively counter ldquo;shift lag.rdquo;Mil Med 2000;165colon;101ndash;105
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