Help Me Make it Through the Night (Shift)

Laura A. Stokowski, RN, MS


January 24, 2012

In This Article

Circadian Disruption

Circadian disruption -- also known as chronodisruption -- is a disturbance of the circadian organization of human physiology, endocrinology, metabolism, and behavior.[17]A master biological clock, located in the suprachiasmatic nuclei of the hypothalamus, controls circadian rhythms generated by feedback loops that involve multiple "clock genes."[18] Core body temperature, blood pressure, sleepiness/wakefulness, mental performance, alertness, and secretion of hormones (such as melatonin, cortisol, prolactin, and growth hormone) are all linked to circadian rhythms.

The major synchronizers of circadian rhythms are exposure to environmental patterns of light and dark.[19] These patterns control biological cycles that repeat roughly every 24 hours (the solar day), and we are entrained to these rhythms. They allow us to have regular oscillations between sleep and wakefulness, and fasting and eating, that are critical to health. When our rest-activity cycles match the light-dark cycles of the environment, we are said to be "in phase." If a person is exposed to inadequate or irregular amounts of light at certain times of the day, circadian rhythm can be disrupted, causing asynchrony between the circadian system and the solar day.[20] This is believed to be the root of long-term negative health outcomes, such as cancer.

If you doubt that working the night shift seriously disrupts circadian organization, consider this: Working a typical night shift schedule creates biological clock stress that is analogous to the jet lag of flying back and forth between Tokyo and San Francisco every few days.[21] It is no coincidence that airline personnel who criss-cross time zones have health consequences similar to those of night-shift workers.[21]

The basis for night shift chronodisruption is exposure to light at night, when humans are supposed to be sleeping. The pineal gland-secreted hormone melatonin is the "messenger of time" that transmits information about environmental light and darkness, obtained from ganglion cells in the retina, through the hypothalamus to all tissues of the body.[15]Melatonin is synthesized and secreted at night, acting as a signal for the length of day and night. Melatonin is also a well-known oncostatic hormone that inhibits tumor growth. Light suppresses melatonin secretion in a dose- (or intensity-) dependent manner. Night sleep normally occurs during the rising phase of melatonin secretion. If a person tries to sleep during the declining phase of melatonin secretion, sleep can be shorter with more awakenings.


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