[Effects of consecutive night shifts with prolonged on-duty and overtime work on various sleep].

The purpose of this research was to investigate the effect of consecutive night shifts upon various types of sleep. The subjects studied were 9 guards who usually worked on an alternate-day shift system. The nighttime nap EEG (3:00-6:30) during the 24-hr on-duty period was recorded, and then the subsequent day sleep EEG (11:00 to spontaneous awakening) and recovery night sleep (23:00 to spontaneous awakening) were also measured. These records were made for the nap, day sleep and recovery night under two different sets of conditions. The first was for the usual shift cycle (U), i.e. 24 hr duty (10:00-10:00)----off duty----24 hr duty----off duty. The other was for consecutive night shifts (CN) with overtime work (18:00-10:00) performed on the normal off day, i.e. 24 hr duty----8 hr off duty----40 hr duty----off duty. The results were as follows: There was no difference between U and CN conditions concerning time in bed and total sleep time in the nighttime nap, day sleep and recovery night. However, the total time of daytime sleep was significantly shortened under both U and CN conditions, as compared with the control night. The amount of REM sleep in the CN condition for both the nighttime nap and day sleep was significantly greater than in the U condition. In the recovery night, REM sleep continued to be slightly greater for the CN condition than for the U condition. Slow wave sleep (SWS) in the CN condition was slightly greater than in the U condition for the nighttime nap and day sleep, but in the recovery night, no difference was observed between the two conditions. In the U condition, the sleep onset REM period (SOREMP) was observed in 2 of the 9 cases, but only during the day sleep. In the CN condition, however, it occurred in 5 subjects during the nighttime nap, in 4 during the day sleep, and in 2 during the recovery night. The REM sleep latency in the CN condition thus tended to be diminished in each type of sleep compared with that in the U condition, but a significant difference between the two conditions was observed only in the nighttime nap. It is believed that the increase in the amount of REM sleep and in the frequency of SOREMP might be due to the repeated pattern of sleep loss and shortened periods of sleep during consecutive night shifts of prolonged duty, with partial sleep deprivation resulting from lack of REM sleep.