RAPID SHIFT IN PEAK MELATONIN SECRETION ASSOCIATED WITH IMPROVED PERFORMANCE IN SHORT SHIFT WORK SCHEDULE

We studied the performance and adaptability of 40 nurses (median age 3 5 years), 20 on permanent day shift and 20 on permanent night shift wi th fast rotation of work and days off, matched for age, gender, and so cio-familial responsibilities. For 15 days prior to the study, subject s maintained sleep logs and trained for performance tests. Questionnai res were administered to evaluate adaptability to shift work. During t he experimental phase, sleep/wake patterns were monitored using sleep logs and activity/inactivity with wrist actigraphy. Performance levels were measured with the four choice reaction time and memory test for seven letters, eight times/day during the wake period, days on and off . On the last day of work and first day off, 6-sulfatoxy-melatonin lev els were assayed from urine samples collected every 2 hours. Estimated total sleep time during the 15-day experimental period was not signif icantly different in the dayshift and nightshift nurses. Night nurses shifted regularly to daytime activities on days off and, as a group, w ere significantly sleep deprived on work days with napping on the job in 9 of the 20 night shift nurses (mean of 114 +/- 45 minutes per shif t) and a significant performance decrement during the work period. Fur ther analysis revealed two subgroups of night nurses: The majority (14 nurses) had a mean peak of 6-sulfatoxy-melatonin at 0718 hours on day s off and no peak during night work while the other 6 night shift nurs es presented a fast melatonin shift with two clear peaks on both work and days off. Comparison of performance scores revealed that all nurse s performed similarly on days off. Daytime nurses and fast-shifting ni ght nurses had similar scores on work days, while nonshifting night nu rses had significantly lower scores at work. Despite similar gender, a ge, social conditions, and light exposure levels, a minority of the nu rses studied possessed the physiological ability to adapt to a fast-sh ifting sleep-wake schedule of more than 8 hours and were able to perfo rm appropriately in both conditions. This shift was associated with a change in the acrophase of 6-sulfatoxy-melatonin.