The biology of getting to sleep
Two major biological processes regulate when we sleep:
1. Our circadian rhythm, or daily sleep-wake cycle: this is our in-built tendency for bodily functions to follow a 24-hour cycle. The timing is co-ordinated by the ‘body clock’, an area of the brain which uses external cues like light and temperature to keep us alert and active during the day, and resting or sleeping at night.
2. Our sleep drive, also called sleep-wake homeostasis: quite simply, the longer we’re awake for, the greater our need for sleep.
The circadian rhythm is usually the strongest factor determining when we sleep, which is why we usually get weary at night time, even if we’ve had a lie in.
The brain produces two main hormones which control the circadian rhythm. Cortisol surges in the morning and helps get us out of bed. Melatonin helps us switch off at night. Melatonin usually starts to be produced at around 8-9pm, peaks at 2-3am (when we get super sleepy) and stops at around 7-8am. We typically also feel sleepy at 2-3pm in the afternoon, when there is a natural lull in our alertness.
Natural levels of alertness through the day
Trying to sleep at the wrong phase of the daily cycle means that even if you’ve been working all through the night and built up a strong sleep drive, you could be trying to close your eyes just as cortisol is sending a biological wake-up call. Shift workers often become anxious about getting to sleep which can lead to insomnia.
Safety risks: lulls in concentration and performance
Studies have shown that sleepiness increases across the night shift. Several studies have shown that the odds of sleep related accidents are at least twice as high for shift workers. In one US study, 1 in 4 police officers admitted falling asleep at the wheel while driving at work.
Shift workers are often deprived of the deep sleep required for memory consolidation and learning. The persistent sleep debt associated with shift work has been likened to the reduced attention and concentration of being intoxicated by alcohol. Alertness is particularly impaired during the transition to night shifts
Long term health risks: a system under stress
It’s not just sleep that follows a 24-hour cycle. Our appetite control, growth and repair, metabolic and circulatory systems all anticipate rest and recovery at night, and alertness during the day.
There is evidence that when we eat during the night, for example, it places more stress on our food processing systems, and we are more likely to develop symptoms of diabetes and weight gain. Shift workers are at greater risk of gastrointestinal problems like diarrhea, constipation and ulcers.
Although some people adapt well to shift work, research shows that in the long term, working night or rotating shifts is associated with increased risks of obesity, high blood pressure, heart attacks, breast cancer and prostate cancer.
Antisocial working hours and sleep deprivation can also take their toll on emotional health. Shift work is associated with daytime fatigue, stress, depression, anxiety and relationship difficulties.