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Body temperature variations throughout the day follow a regular cycle. The highest temperature occurs around 1700 hours and the lowest at about 0500 hours, at which time we are least efficient and the desire for sleep is at its peak.

Timing Planned Sleep

Time spent awake is important in determining readiness for sleep but there is also a circadian rhythm of sleep. This means that at certain times of the day even the sleep-deprived individual may have difficulty in falling asleep. It is the timing of sleep not the amount of time awake that is the critical factor in determining sleep duration. As indicated earlier, the duration of sleep is linked to the body temperature cycle.

Sleep taken at times near the temperature peak or when the temperature is falling will be longer and more refreshing than sleep taken when body temperature is rising. Aircrew attempting to sleep when the body temperature is on the rise will have considerably more difficulty getting to sleep, and if successful, will usually awaken within a relatively short period of time.

Time of Day and Performance

As well as the circadian rhythms of temperature and other basic physiological processes, there are rhythms for more complex behaviours. Performance of different tasks is affected by the time of day. Simple tasks, requiring little short-term memory input, follow the pattern of body temperature. Performance improves as temperature increases and declines as the temperature decreases. Performance using short-term memory tasks declines throughout the day. Verbal reasoning and mental arithmetic skills peak around midday.

Accident statistics have been examined to detect a correlation between time of day and accidents. It has been found that driving accidents peak at certain times of the day, for example 1500 hours, but other factors, such as traffic density and road conditions will also affect the results.

With regards to aviation accidents, the time of day has been noted as a causal factor in a number of incidents.

Credit/Debit Systems

General

The sleep/wake cycle can be thought of as a credit and debit system. In this system the individual is given two points for every hour spent asleep and has one point deducted for every hour spent awake. This is only a rough measure, as individuals vary considerably in the amount of sleep they require.

Sleep Credit Limit

The maximum credit available is 16 points. You cannot store credit points above 16 in anticipation of a long period of awake. A sleep of 10 to 12 hours after a long period of strenuous activity will only give the 16 credits and the individual will feel sleepy again after 16 hours, not after 20 to 24 hours.

However, if a period of wakefulness is significantly foreshortened (the individual is still in a state of sleep credit) then a good sleep is unlikely.

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Sleep Debit

The fewer points you have the readier you are to sleep. Normally the person will sleep when he/she has little or no credit and will sleep for about eight hours, followed by a wakeful period of about sixteen hours when the sleep credit will be exhausted. A gradual reduction in level of credit may build up over a period of time as a ‘cumulative sleep debt’. It is important to realize performance reduction, resulting from sleep deprivation, increases with altitude.

Measurement and Phases of Sleep

(see Figure 11.3)

Measurement

Laboratory experiments have revealed a great deal about the various sleep phases. Volunteers have undergone a number of measurements and observations whilst they are asleep. The devices used include:

•Electroencephalogram (EEG) - to record the electrical activity of the brain

•Electrooculogram (EOG) - to measure eye movement within the eye socket

•Electromyogram (EMG) - to measure muscle tension or relaxation

Sleep Stages

(see Figure 11.3) The stages of sleep are classified into 4 stages:

Stage 1

The sleeper is in a very light sleep. It is a transitional phase between waking and sleeping; if woken at this stage the volunteer may claim that he has not even been asleep. In early sleep we pass through about 10 minutes of stage 1 before moving to the deeper stage 2.

Stage 2

In early sleep we spend about 20 minutes in stage 2 before moving on to the deeper stages 3 & 4. About 50% of a normal sleep is spent in stage 2.

Stages 3 & 4

During Stages 3 & 4 sleep:

•The brain is semiactive emitting long slow waves measured by EEG tracings and thus it is commonly referred to as ‘slow wave’ or orthodox sleep.

•The eyes are stationary behind the eyelids.

•The muscles are relaxed.

•Choking or crushing dreams.

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Figure 11.3 Sleep profile for a typical night’s sleep

Function of SlowWave Sleep (Orthodox Sleep)

Slow wave sleep refreshes the body and is necessary for tissue restoration. After strenuous physical activity the body will require more slow wave sleep.

Rapid Eye Movement (REM) Sleep

Superimposed on the above 4 stages is REM (sometimes referred to as paradoxical sleep) which is quite different from orthodox sleep. In this phase:

•The brain is active and the EEG trace is similar to that of an individual who is fully awake whilst the other measurements show the person to be asleep.

•Rapid eye movement behind the eyelids are detected.

•Whereas there is near total muscle paralysis (thought to prevent the sleeper acting out dreams), there is frantic movement of the muscles of the eye. This motor activation occasionally breaks through resulting in twitching of the limbs.

•Complex, bizarre, and emotionally-coloured dreams take place.

Function of REM Sleep

REM sleep refreshes the brain. It strengthens and organizes the memory. After a period of learning new tasks or procedures REM sleep will increase. In addition, REM sleep contributes significantly to emotional equilibrium and good humour. Thus, irritability normally follows a period of disrupted sleep.

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Characteristics of Orthodox and Paradoxical Sleep

Some characteristics of orthodox and paradoxical sleep are:

Characteristic	Orthodox Sleep	Paradoxical Sleep
EEG (brain waves)	Large slow waves	High frequency
EOG (eyes)	Still	Rapid eye movements
EMG (throat)	Relaxed muscles	Tensed muscles
ECG (heart)	Regular	Irregular
Dreaming	Normally no recall	Recall
Sleep walking	Yes	No
Body movements	Less frequent	More frequent
Stomach acids	Steady	Increase
Function	Tissue restoration	Memory organization

Sleep Cycles

During any normal night’s sleep the pattern operates on an approximately 90 minute cycle. Towards the end of the first 90 minutes of falling asleep the first REM stage occurs but this first REM experience lasts only 10 to 20 minutes before the person passes back into slow wave sleep.

At the end of the second cycle of 90 minutes the duration of REM sleep periods increases.

Sleep Profile

A sleep profile for a typical night’s sleep is shown in Figure 11.3. The individual stages will vary depending on the activities prior to sleep. If a great deal of strenuous physical activity has taken place then the sleep stages 3 and 4 will be extended. Alternatively, if a lot of mental work has been undertaken, such as learning new information or procedures, then REM sleep will be increased.

Rebound Effect

Sleep deprivation experiments have shown that if a person is deprived of either slow wave or REM sleep there will be a ‘rebound’ effect in the next sleep period. That is the individual will make up the deficit in either case. For example if one is woken after 3 hours of a normal sleep period then the body will have had all its required slow wave sleep, but be deficient in REM sleep. In the next sleep period it is found that REM sleep will occur earlier and last longer than normal.

Age and Sleep

Individuals differ in the amount of sleep they require. In a survey of one million people the most frequently reported sleep duration was between 8 and 9 hours. Some people seem able to do with much less sleep and can manage quite well on 3 to 4 hours per night.

Ageing brings major changes in sleep requirements. New born babies may sleep for up to 23 hours per day (of which the majority is REM) and even as they grow older will require much

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