NIOSH Training for Nurses on Shift Work and Long Work Hours

Circadian Rhythms

Circadian rhythms have an important purpose: they prepare your body for expected changes in the environment and, for example, the time for activity, time for sleep, and times to eat.

• External cues are important; the strongest is the sun’s light/dark cycle. Artificial light also influences the pacemaker.
• Circadian rhythms need time to adjust to new sleep times, so changing work times can be difficult.
• In general, if people have to change their sleep times (for example, for work or travel), they tend to have more difficulties getting up earlier and have an easier time getting up later. This is because the circadian pacemaker tends to run longer than 24 hours, which makes it easier to sleep later in the morning and go to bed later.

The sleep and work schedules of shift workers are frequently in conflict with the sun’s light/dark cycle, which drives their circadian rhythms to promote sleep at night and wakefulness during daylight hours. As a preview of information to come in Part 2 of this training program, there are strategies that help promote adjustment of the circadian rhythms so a worker can better adapt to working at night.