Chapter

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Sleep and Fatigue

General . . . . . . . . . . . . . .	. . . . . . .	. .	. .	. .	. .	.	.	205
Biological Rhythms and Clocks								205
Body Temperature . . . . . . . . . .	. . . . . . .	. .	. .	. .	. .	.	.	206
Time of Day and Performance . . . . . .	. . . . . . .	. .	. .	. .	. .	.	.	207
Credit/Debit Systems . . . . . . . . . .	. . . . . .	. .	. .	. .	. .	. .		.207
Measurement and Phases of Sleep . . . .	. . . . . . .	. .	. .	. .	. .	.	.	208
Age and Sleep								210
Naps and Microsleeps								211
Shift Work . . . . . . . . . . . . . .	. . . . . .	. .	. .	. .	. .	. .		.211
Time Zone Crossing . . . . . . . . . . . . . . . . . . . . . . . . .		. . .	. . .	. . .	. . .	. . . .212
Sleep Planning								214
Sleep Hygiene . . . . . . . . . . . .	. . . . . . .	. .	. .	. .	. .	.	.	215
Sleep and Alcohol								216
Sleep Disorders . . . . . . . . . . . .	. . . . . .	. .	. .	. .	. .	. .		.216
Drugs and Sleep Management								217
Fatigue								217
Vigilance and Hypovigilance . . . . . . .	. . . . . . .	. .	. .	. .	. .	.	.	218
Questions . . . . . . . . . . . . . .	. . . . . .	. .	. .	. .	. .	. .		.220
Answers . . . . . . . . . . . . . .	. . . . . . .	. .	. .	. .	. .	.	.	224

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General

Introduction

Sleep is essential to human well-being. During a sleep period the body is not only recuperating from the physical activity of the day but it is also carrying out essential organization of the mental processes. The amount of sleep required varies according to age, amount of physical and mental energy used prior to sleep and individual differences. Sleep exhibits particular cycles during each sleep period, varying from light dozing to very deep sleep, with intervals of a unique type of sleep in which vivid dreams occur. The duration of sleep and its quality depends to a large extent on our internal body rhythms, and it is well to consider these rhythms before looking at sleep itself.

Aircrew’s Attitude to Sleep

Aircrew must not regard sleep as merely a mechanism for recuperation from the previous day’s activity. It is of fundamental importance that aircrews’ attitude towards sleep is proactive and that sleep is actively planned in order that flights are conducted at maximum physical and mental efficiency.

Biological Rhythms and Clocks

Introduction

Many physiological processes in the body undergo rhythmic fluctuations (through the hypothalamus in the brain), and these occur whether the person is awake or asleep. These rhythms are controlled internally and are not simply reaction to our environment. The rhythms are not necessarily synchronized and these fluctuations or cycles can vary from about 90 minutes to as much as 28 days. The discipline of studying rhythms is called chronobiology.

Circadian Rhythms

The most common rhythms exhibited by man and most other animals have periodicities of about 24 hours and are known as circadian rhythms. These rhythms are seen as measurable and regular daily fluctuations - sometimes greater than 50% of the daily mean - in variables such as:

•Body temperature.

•Blood pressure.

•Heart rate.

•Sensory acuity.

•Adrenal gland output.

•Brain neurotransmission levels.

In normal conditions our circadian rhythms are locked to 24 hours by external time cues (Zeitgebers).

Zeitgebers

These cues are provided by clock times and other external events, such as the sun rising, light and darkness, the increase in traffic noise at certain times, regular mealtimes, and work schedules all of which assist in the regulation of our internal biological clock. These cues are known as zeitgebers (German for ‘time givers’). In fact, cognitive awareness of the clock time is, in itself, an example of a zeitgeber.

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If an individual is isolated from these zeitgebers, without clocks, set meal times, or any way of detecting light changes, the circadian rhythms will ‘free run’ to a periodicity of about 25 hours. This means that an average individual, if isolated from these cues, instead of working to an average 16 hours awake and 8 hours sleep, will extend his/her day to 17 hours awake, 8 hours sleep.

Fatigue and Sleep 11

Figure 11.1 A sleep pattern on successive days without zeitgeber cues to time

Body Temperature

BodyTemperature and Sleep

There is a direct relationship between our body temperature and sleep cycle. At the time of lowest body temperature we find it hardest to stay awake. We will start to feel sleepy at a time when the temperature is falling and be at our most wide awake when the temperature is rising. This relationship explains the difficulty we may have of sleeping well for a few days after time zone crossings. This is one of the symptoms of ‘jet lag’.

Figure 11.2 The circadian rhythm of body temperature

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