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Shift work and The Body Clock

There are lots of different work patterns that people identify as shift work.  From a physiological perspective, shift work is any work pattern that requires you to be awake when you would normally be asleep, on a day when you're free to choose your own schedule.


The circadian body clock is a pacemaker in the brain, which governs daily (circadian) rhythms in how we function (for example, metabolism, some hormone production, and our ability to carry out physical and mental work) and how we feel (for example, our level of sleepiness, fatigue and mood).

The circadian body clock needs to keep in step with the day/night cycle and it does this by tracking light intensity. It can do this even through closed eyelids. It has its own receptor system in the retina (separate from vision) and is particularly sensitive to blue light.

Through the circadian body clock, we are programmed to sleep at night. Our sleep quality and structure is optimal, and sleep is most refreshing and restorative, when we sleep at night.


Shift work requires trying to override the circadian body clock's preference for sleep at night. Unfortunately, the clock cannot fully adapt to shift work, even permanent night shift work, because it is always being drawn back to its preferred pattern of sleep at night by the day/night cycle (exposure to light) and the rest of day-active society (e.g., social interaction, meal times etc.). People also tend to go back to sleeping at night on their days off.

The circadian body clock has outputs to sleep-promoting and wake-promoting centres in the brain, which result in times when it is easy to sleep and times when it is hard to sleep. These are summarised in the following figure.

This means that shift workers are often trying to work at times when they are least functional and most likely to make errors, and to sleep at times when they are programmed to be awake.


Compared to nurses who work day or evening shifts, nurses who work rotating shifts are twice as likely to report clinical errors and have a motor vehicle accident.  Compared to nurses working shorter shifts, nurses working extended shifts (>8 hours) are 2-3 times more likely to make clinical errors, have poorer patient outcomes, including increased patient mortality and feel drowsy while driving.  They have a greater likelihood of injuring themselves and others, and are more likely to report poor quality care and poor patient safety.

This project aims to develop tools to better manage fatigue so that these risks can be reduced.


The Nurses’ Health Study is a prospective study of 74,862 registered US nurses that has shown remarkable retention of participants (90% follow-up at 22 years).  The Study has yielded a wealth of data including for the evaluation of the impact of shift work on long term health.  

After controlling for a number of risk factors, rotating night shift work has been shown to be an independent predictor of: 

  • increased all-cause and cardiovascular disease mortality (after ≥5 years exposure to shift work).
  • increased lung cancer mortality (after ≥15 years exposure).
  • a 4% increased risk of ischemic stroke for every 5 years of additional exposure (after ≥15 years exposure).
  • a monotonic increase in risk of type 2 diabetes with increasing exposure, that appears to be partly mediated through body weight.

More fatigue facts

Fatigue at work

The importance of sleep

Fatigue countermeasures