Sleep
Sleep is the resting state in which the body is not active and the mind is unconscious. Vice versa, lack of sleep is a lack of that type of rest and sleep debt is its deficit.
Description
According to the CAA Flight-crew human factors handbook CAP737,
Sleep is essential for restoring the normal balance between the different parts of the central nervous system. It may be thought of as being analogous to the way a computer might work: transferring the day's experiences (from the preceding period of wakefulness) from RAM (random access memory – the type of memory used in the moment) onto the hard disk. This frees up the RAM for use during the next period of work (wakefulness).
Also during sleep, the body's physical functions are rested and some renewal takes place. During sleep, sympathetic nervous activity decreases and the muscular tone becomes almost nil. The arterial blood pressure falls, the pulse rate decreases, the blood vessels in the skin dilate and the overall basal metabolic rate of the body falls by up to 20%.
On average, most humans physiologically need about 8 hours of sleep per night. However, in modern society most adults report an average of 7 and 7.5 hours sleep per night. Studies have shown that up to 75% of adults report day time sleepiness, with nearly a third of them reporting severe levels which interfered with activities.
Sleep loss can be acute or cumulative. In an acute situation, sleep loss can occur either totally or as a partial loss. It can accumulate over time into what is referred to as "sleep debt." As little as 2 hours of sleep loss can result in impairment of performance and levels of alertness. Sleep loss leads to increased reaction time, reduced vigilance, cognitive slowing, memory problems, time-on-task decrements and optimum response decrements. It has also been shown that performance variability increases with sleep loss.
The normal sleep requirement is 8 hours in every 24-hour period, and it is possible to perform a simple calculation of sleep debt when this is not achieved. As the sleep requirement is 8 hours, within a 24 hour period this leaves 16 hours available for activity. Alternatively, this can be expressed as one sleep hour being good for two hours of wakeful activity. The maximum possible credit to offset against sleep debt is 8 sleep hours, meaning that no matter how long is spent sleeping, sleep debt will still begin occur after about 16 hours of continual wakefulness. It is not necessary to sleep for the exact number of hours in deficit to recover sleep debt. Most people find that a 'catch-up' sleep of around one third of the deficit is sufficient. Sleep debt can become cumulative, leading to a decrement in alertness and performance if the deficit is not recovered within a reasonable time.
Stages of sleep
According to the CAA Flight-crew human factors handbook CAP737,
Sleep can be divided into five stages - stages 1 to 4 and rapid eye movement (REM) sleep. Stage 1 is a transitional phase between waking and sleeping and this normally takes around 10 minutes as a person falls asleep. Sleep then becomes deeper with 15 minutes in stage 2 sleep and a further 15 minutes in stage 3 sleep before moving on to stage 4. Approximately 90 minutes after sleep onset, REM sleep will occur. The cycle of REM sleep and stage 1 to 4 sleep repeats during the course of the night in 90 minute cycles, each succeeding cycle containing greater amounts of REM sleep. An 8 hour sleep period will typically contain about four or five bouts of REM sleep. Most stage 4 sleep happens early in the night, as can be seen in the hypnogram in which the solid bars represent REM sleep.
The stages of sleep can be shown by examination of the electroencephalogram (EEG), which measures electrical activity in the brain. The pattern changes for each stage, and stages 3 and 4 are referred to as 'slow wave' sleep from the shape of the EEG tracing. During an eight-hour period of sleep, 50% is usually made up of stage 2.
It is thought that the stage 1 to 4 sleep is related to body restoration whereas as the REM sleep may be related to strengthening and organising memory. When learning new tasks, an increased proportion of REM sleep is seen. REM sleep is sometimes referred to as 'paradoxic sleep'.
