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[Excerpt] Today, only about 30% of employed Americans work during the daytime, 35 to 40 hours a week, five days a week, Mondays through Fridays. When people 8-hour rotating work shifts, with their associated tightly-packed schedule, shift workers work 400 more hours per year than those who work only 40 day-time hours.

Our circadian (“around the day”) rhythm is the result of our brain and body blending internal body cycles to be in sync with the external world. We are aware of the time of day; light/dark (day/night); mealtimes, traffic noise, and what everybody else is doing. There are separate high and low activity periods throughout the 24-hour day for our internal cycles of hormones, heart rate, body temperature, blood pressure, etc. Our body temperature drops to its lowest point around 3-4 AM, then rises slowly again at about 5-6 AM – this affects performance, activity, and alertness and is the most difficult time to stay awake and alert. About 10 – 20% of the population seem to naturally be “morning types” (“larks”) and “evening types” (“owls”). Morning types have a small swing between their body temperature maximum and minimum and seem to have more trouble adjusting to shift work than the evening types with a relatively large swing in body temperature.

While our systems can adjust without difficulty to small, gradual changes -- such as seasonal changes in day length – the abruptness of changing shifts causes our bodies to become temporarily and severely disorganized. Once disrupted, the body tries to adapt by re-synchronizing all the affected functions, but these take different amounts of time to reach the new rhythm. If the cycle is disrupted again before a new rhythm is in place, the body needs to adjust again. Moreover, work schedules may require sleeping during the day – this can be difficult with daylight and daytime noises – so people sleep fewer hours and are less refreshed. Sleep deprivation is a serious problem because we need sleep to accomplish important brain activities, some requiring a lot of energy and so can’t be done while we’re awake and using this energy to process sensory information. Most adults need about 8 hours of sleep, though the typical range is 6 to 10 hours. Unfortunately, you can’t make up a sleep debt, even if you try to sleep late on your days off.


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© Cornell University. Reprinted with permission. All rights reserved.

Recommended Citation
Brown, N. J. (2019). Risks and risk reduction for shift work, long hours of work, and sleep deprivation [Electronic version]. Ithaca, NY: Cornell University, Workplace Health and Safety Program.