Ma. Querasalva et al., RAPID SHIFT IN PEAK MELATONIN SECRETION ASSOCIATED WITH IMPROVED PERFORMANCE IN SHORT SHIFT WORK SCHEDULE, Sleep, 20(12), 1997, pp. 1145-1150
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.