NIOSH Training for Nurses on Shift Work and Long Work Hours

Circadian Rhythms

Circadian rhythms are internally driven cycles of biochemical, physiological, and behavioral processes of living beings that rise and fall across the 24-hour day. There are numerous peripheral circadian oscillators (clocks) throughout the human body that drive circadian rhythms. A master circadian pacemaker^a (sometimes called the circadian clock) in the hypothalamus of the brain (see Figure 2.4) synchronizes and controls the timing of these peripheral circadian oscillators so they work together.

Circadian rhythms promote sleepiness before usual bedtime, help initiate sleep, and begin promoting wakefulness before usual wake-up time in the morning. The pacemaker has an internally driven 24-hour rhythm that tends to run longer than 24 hours but is reset every day by external timing cues to keep the cycle at 24 hours. The light/dark cycle of the sun is the strongest cue.²⁵ Exercise and melatonin also appear to influence the timing of the pacemaker, but not as much as light. Normally, light enters the eye (even through closed eyelids during sleep) and signals to the pacemaker the time for awakening and activity and the time for sleep (see Figure 2.4).

As well as sleep and wakefulness, the pacemaker sets the timing for numerous circadian rhythms that regulate physiological and behavioral processes, such as those related to the following:

• Hormones
• Body temperature rhythm
• Eating and digesting food

^aTwo small groups of nerve cells that lead numerous circadian rhythms.