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The Science of Fatigue

Glen Ridgway - independent consultant @ Ridgway Workplace Mental Health

Fatigue is more than just feeling tired. It is a decline in our mental and/or physical performance that results from prolonged exertion, sleep loss and/or disruption of the internal clock.

Fatigue can lead to errors and accidents, ill-health and injury. It cannot be ‘cured’ by a quick nap or even a single good night’s sleep.

Fatigue is a known risk is some occupations – especially those that require shift work and as such it needs to be managed.

To enable us to manage fatigue we need to understand the science behind it so we can ensure any action we take will be effective.

In simple terms fatigue can be considered as an imbalance between

· the physical and mental exertion of all waking activities; and

· recovery from that exertion.

It follows that managing fatigue requires strategies to manage the exertion (only some of which will be work related) and/or opportunities to recover.

Two areas of science are central to this:

1. Sleep science — particularly the effects of not getting enough sleep either in one night or over several nights.

2. Circadian rhythms — daily cycles in physiology and behaviour driven by our internal ‘body clock’.

By looking at these in more detail we will be able to create some scientifically based principles that will allow us to develop effective fatigue management procedures.


Most people know that there are two main types of sleep:

· Rapid Eye Movements (REM) sleep. During this time the brain is restoring itself, processing information from the previous day’s activities and getting itself ready to face the next day.

· Non-REM Sleep, which is where the brain activity gradually slows in stages into Deep Sleep.

It is during Non-REM sleep that the body recharges and repairs itself.

During a night’s sleep we cycle between non-REM sleep and REM sleep. An 8-hour sleep usually lasts four cycles and a for sleep to be fully restorative, it must contain unbroken cycles of non-REM and REM sleep.

There are several factors that can affect the quality of our sleep. These include: Caffeine and nicotine which stimulate the brain, making it harder to fall asleep; Alcohol, while the body is processing alcohol, the brain cannot obtain REM sleep.; Age; Sleep disorders and environment.


Losing just two hours sleep on one night will reduce alertness and degrade performance the next day. Studies show that the effects of restricting sleep night after night accumulate; this is known as ‘sleep debt’. As the sleep debt increases, we become progressively less alert and less able to function.

The effects of sleep debt can affect us for several weeks, and recovery is not simply a case of swapping an hour of lost sleep with an hour of sleep later. Studies show that it takes at least two consecutive nights of unrestricted sleep are required for the non-REM/REM sleep cycle to return to normal.

If sleep debt is prolonged, recovery is a slow process that can take a couple of weeks and will require longer periods of time off


Circadian rhythms are physical, mental, and behavioural changes that follow a daily cycle, and which are controlled by our internal body clock. There is lots of exciting science being done around how circadian rhythms affect different aspects of our body and mind, but for this article the most important aspect is sleep/wake rhythm; a fixed biological system that is running in the background of your brain and cycles between sleepiness and alertness at regular intervals.

Most adult’s greatest need for sleep is somewhere between 2:00am and 4:00am when our energy levels are at their lowest. Most of us will also have experienced post lunch sleepiness, when our energy level drops again and we fancy a short nap.

Circadian rhythms have evolved to optimise our body’s ability to recuperate, and it is when our waking and working habits conflict with these rhythms that problems occur. Have you ever noticed how much harder it is to stay awake after lunch if your sleep has been disturbed the night before?


From the perspective of human physiology, shift work can be defined as any work pattern that requires us to be awake during the time in the circadian sleep/wake rhythm when we would normally be asleep.

We are awake when the body says we should be asleep, and we are trying to sleep when the body says we should be awake. No wonder our sleep is disturbed, and we get fatigued.

Evidence shows that after several weeks the circadian rhythm can ‘shift’ but only by a couple of hours. Never by the 12 hours it would need to for us to become truly nocturnal. Night workers do get used to an irregular sleep/wake cycle after a few weeks, but at lower than normal alertness and performance levels.


Workload can be described as a ‘mental and/or physical activity’ and can include a potential to cause fatigue.

Workload can be looked at in two ways

1. The nature and amount of work to be done (including duration, difficulty, complexity and intensity of the task).

2. Capability of person doing the task (including experience and skill level, sleep history, and circadian phase, current levels of fatigue).

A high workload, which is physically or mentally demanding will increase levels of fatigue and require longer to recover. If we are already fatigued, we will be less able to achieve a high workload and we will become more fatigued more rapidly.

High workload may also have consequences for sleep, due to the time required to “wind down” after demanding work.


If we understand the science of fatigue, we can develop some guiding principles that can help us develop effective fatigue management processes.

· Getting enough sleep (both quantity and quality) on a regular basis is essential for restoring the brain and body.

· A single night of disturbed sleep decreases alertness, performance and increases sleepiness the next day.

· Circadian rhythms affect the timing and quality of sleep. This is worsened during shift work when the need for sleep and the opportunity to sleep are out of sync.

· Workload can contribute to an individual’s level of fatigue

As individuals we have a responsibility to maintain our own fitness for work, but as employers we have a duty to recognise fatigue risk, raise awareness of the topic and develop procedures and processes that effectively eliminate, mitigate or manage fatigue in our employees.

Night work and shift work are a reality for many people, but by understanding the science of fatigue we can develop work patterns that provide enough recovery time, help maintain alertness performance levels and keep our people safe.

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