NIOSH Training for Nurses on Shift Work and Long Work Hours
Desynchronization of Homeostatic and Circadian Processes
Figure 2.6b. Working night shift leads to loss of synchronization of the homeostatic sleep pressure drive and circadian wakefulness rhythm.
If a night shift worker’s circadian system has not adjusted to working at night and sleeping during the day, the sleep pressure drive and circadian rhythms for wakefulness will not be synchronized and, as a result, will not work together. As shown in Figure 2.6b, the circadian rhythms that drive wakefulness will continue to rise during the daytime, pushing the person to be awake, and will fall at night, pushing the person to go to sleep. The sleep pressure drive will increase while the worker is awake. As a result, night workers tend to fall asleep quickly in the morning after their work shift, because they have high pressure for sleep from being awake a long time (as shown on the graph). But their sleep can be disturbed and they can have difficulty remaining asleep long enough because circadian rhythms that drive wakefulness are increasing and pushing them to wake up. Noise or minor discomfort can awaken them more easily because of the rising drive for wakefulness. On awakening, they may not feel refreshed, but can have difficulty going back to sleep. Because of insufficient sleep, the sleep pressure drive will not decrease fully (dashed line in Figure 2.6b indicates usual fall in sleep pressure drive after a good night’s sleep). This may lead to sleepiness and fatigue when awake. When working at night, a worker’s circadian rhythms for wakefulness decrease, causing strong feelings of sleepiness, especially between 2 a.m. and 6 a.m. At the end of the shift, night nurses are at maximum risk for sleepiness from both high sleep pressure and low circadian drive for wakefulness. This is the time for maximum risk of vehicle crashes due to drowsy driving which endangers the nurse as well as other people on the roads.