Facts about Sleep and Fatigue

Circadian Rhythm

Circadian rhythms are physiological cycles that follow a daily pattern. We are "programmed" by our circadian rhythms to sleep at night and to be awake during the day.

During nighttime hours and to a lesser extent during afternoon "siesta" hours, most types of human performance are significantly impaired, including our ability to drive.

Problems occur if we disrupt our natural sleep cycles (e.g. by staying awake during the night), do not get enough sleep, or get poor quality sleep.

Circadian rhythms cannot be reversed. Even if you have been working nightshifts for many years, your body will still be programmed to sleep at night.

Sleep Debt

The human body requires a certain amount of sleep each night to function effectively. The average amount of sleep a person needs is 8 hours. When we reduce the number of hours we sleep at night we start to accumulate what is called a 'sleep debt'.

Sleep debt is defined as the difference between the hours of sleep a person needs and the hours of sleep a person actually gets.

For example, if a person needs 8 hours of sleep per night but only gets 6 hours of sleep one night, they have a sleep debt of two hours. These lost hours of sleep need to be replaced.

When we have sleep debt, our tendency to fall asleep the next day increases. The larger the sleep debt, the stronger the tendency to fall asleep.

Sleep debt does not go away by itself. Sleeping is the only way to reduce your sleep debt.

Sleep Inertia

Sleep inertia is the feeling of grogginess after awakening and temporarily reduces your ability to perform even simple tasks.

Sleep inertia can last from 1 minute to 4 hours, but typically lasts 15-30 minutes.

The severity of sleep inertia is dependent on how long you have been asleep and the stage of sleep at awakening. Effects can be severe if a person is very sleep deprived or has been woken from a deep sleep stage. However, sleep inertia can usually be reversed within 15 minutes by activity and noise.

Sleep inertia can cause impairment of motor and cognitive functions and can affect a person's ability to drive safely. Sleep inertia can be very dangerous for people who drive in the early morning hours and shortly after waking up from a sleep.

Microsleeps

Microsleeps are brief, unintended episodes of loss of attention associated with events such as blank stare, head snapping, prolonged eye closure, etc., which may occur when a person is fatigued but trying to stay awake to perform a monotonous task like driving a car or watching a computer screen.

Microsleep episodes last from a few seconds to several minutes, and often the person is not aware that a microsleep has occurred. In fact, microsleeps often occur when a person's eyes are open.

While in a microsleep, a person fails to respond to outside information. A person will not see a red signal light or notice that the road has taken a curve.

Microsleeps are most likely to occur at certain times of the day, such as pre-dawn hours and mid-afternoon hours when the body is "programmed" to sleep.

Microsleeps increase with cumulative sleep debt. In other words, the more sleep deprived a person is, the greater the chance a microsleep episode will occur.

In one study of microsleep, participants were asked to press a button when a strobe light was flashed directly in their eyes every few seconds. During a microsleep they did not notice the light and were not even aware that they had been asleep.