Sleep and Circadian Rhythms

Sleep is the body's recurring period of reduced consciousness and activity, during which the body and brain recover. It is not a uniform state but a structured sequence of stages repeated through the night. When sleep can be taken, and how refreshing it is, depend on circadian rhythms, the internal cycles of about 24 hours that raise and lower body temperature, hormone levels and alertness.

For pilots the subject is practical. Early reports, night duties, time-zone crossings and irregular rosters all push sleep into the wrong part of the body's day or cut it short, and the result is fatigue. Understanding how sleep is built and how the body clock works explains why the rules on flight time limitations look the way they do, and how crews can make the most of the rest they are given.

Measuring sleep

Sleep is studied in the laboratory by recording several signals at once, a technique called polysomnography. EASA human performance texts name three:

• the electroencephalogram (EEG), which records the electrical activity of the brain;
• the electrooculogram (EOG), which records movements of the eyes in their sockets;
• the electromyogram (EMG), which records the tension or relaxation of the muscles.

The heart (ECG) is commonly recorded too. The combination of brain waves, eye movements and muscle tone identifies each stage of sleep.

Chronobiology is the study of biological rhythms. EASA texts note that many of them are driven through the hypothalamus in the brain and continue whether the person is awake or asleep.

Sleep stages and cycles

The pattern of stages through a night is called sleep architecture. The ATPL texts divide sleep into four stages plus REM sleep:

• Stage 1, the transition from waking, is light and brief, lasting about 10 minutes early in the night.
• Stage 2 lasts about 20 minutes early in the night and makes up roughly half of total sleep time.
• Stages 3 and 4 are deep slow-wave sleep (SWS), also called orthodox sleep.
• REM sleep follows at the end of a cycle.

Since 2007 the scoring rules used in sleep laboratories merge stages 3 and 4 into a single stage, N3, so modern medical texts count three non-REM stages plus REM. The exam texts keep four.

Sleep runs in a sleep cycle of about 90 minutes. The first REM period comes towards the end of the first cycle and lasts only 10 to 20 minutes; later REM periods grow longer. Deep slow-wave sleep dominates the first cycles and REM the last ones, towards morning. A normal night contains several cycles, so cutting it short removes mainly REM sleep.

REM and slow-wave sleep

Slow-wave sleep refreshes the body. The brain is only partly active and emits long, slow waves, the eyes are still and the muscles relaxed. It is needed for tissue restoration, and more of it is taken after strenuous physical activity.

Rapid eye movement (REM) sleep, also called paradoxical sleep, refreshes the brain. The EEG looks like that of a fully awake person, the eyes move rapidly behind the closed lids and the other muscles are almost completely paralysed, which prevents dreams from being acted out; complex, emotional dreams occur in it. REM sleep strengthens and organises memory, increases after new tasks or procedures have been learned, and supports emotional equilibrium, which is why irritability follows disrupted sleep.

The body keeps account of both. If a person is deprived of either slow-wave or REM sleep, the next sleep period makes up the deficit: the missing stage starts earlier and lasts longer than normal, an effect known as REM rebound when it concerns REM sleep. EASA texts also note that the performance loss from sleep deprivation increases with altitude.

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Circadian rhythms and zeitgebers

A circadian rhythm has a period of about 24 hours. Body temperature, blood pressure, heart rate, sensory acuity and the output of the adrenal glands all follow one. EASA texts place the highest body temperature at about 17:00 and the lowest at about 05:00; the low point is when efficiency is poorest and the desire for sleep strongest. Sleep begun while body temperature is high or falling tends to be longer and more refreshing than sleep begun near the low point, one reason why daytime sleep after a night duty is usually short and light.

The hormone melatonin, which the body secretes mainly in the evening and through the night, is part of the same system.

The internal clock does not run at exactly 24 hours. Isolated from all time cues, a person's rhythm becomes a free-running rhythm with a period longer than a day. The early isolation studies found about 25 hours, the figure used in ATPL texts; later studies under controlled lighting put it nearer 24.2 hours. Either way the clock tends to drift later.

It is held to 24 hours by zeitgebers (German for "time givers"): daylight above all, and also meal times, clock time, social contact, work schedules and even traffic noise.

Window of circadian low

Alertness and performance are lowest in the early hours of the morning, when core body temperature is at its minimum. Flight time rules formalise this as the window of circadian low (WOCL). EASA (ORO.FTL.105) defines it as 02:00 to 05:59 in the time zone to which the crew member is acclimatised, and the FAA (14 CFR 117.3) uses the same hours. Duty that starts in or runs through the WOCL is given shorter maximum flight duty periods (see flight time limitations).

A smaller dip in alertness occurs in the early afternoon, the post-lunch dip. Training texts note that it occurs whether or not a meal has been eaten, although a heavy meal adds to the drowsiness.

Jet lag and acclimatisation

Jet lag, or circadian dysrhythmia, is the disruption that follows a rapid crossing of time zones, when the body clock no longer matches local time. Sleep comes at the wrong time and is poor, alertness is low when it is needed, and different body functions readjust at different rates, so for a while they are out of step with each other as well as with the clock on the wall. Repeated disruption may cause stomach and bowel disorders.

Direction matters. Because the free-running clock tends to run long, a westbound flight, which lengthens the day, works with it, while an eastbound flight shortens the day and forces the clock forward, so eastbound adjustment is usually harder. EASA ATPL texts use a rough planning figure of about 90 minutes of resynchronisation a day, so a nine-hour shift such as London to Los Angeles takes about six days; it is only an average, and the real rate varies between people, between body functions and with the direction of travel. A crew that returns home after two or three days never fully adapts, and its clock is disturbed again on the way back. For stopovers of less than 24 hours the same texts advise keeping to home-base time for sleep and meals; for longer stopovers, adapting to local time as soon as possible.

Flight time rules deal with this through acclimatisation. EASA defines an acclimatised crew member as one whose circadian clock is synchronised to the time zone where he or she is, and treats a crew member as acclimatised to a 2-hour-wide time zone around the local time at the point of departure. After time-zone crossings, a table in ORO.FTL.105 uses the time difference and the time elapsed since reporting to decide whether the crew member is still acclimatised to the original time zone, has become acclimatised to the new one, or is in an unknown state. The FAA's Part 117 treats a flightcrew member as acclimated after 72 hours in a theater, an area in which departure and arrival points differ by no more than 60° of longitude, or after at least 36 consecutive hours free from duty.

Shift work

Rosters that alternate early, late and night duties impose the same kind of disruption without any travel. EASA texts recommend that shift rotation move to later shifts, from early to late to night, because this follows the clock's natural tendency to drift later. Night workers sleep in the day, when the clock favours waking, so their sleep is shorter and lighter; a dark, quiet room and a planned nap before a night duty help. Caffeine can be used tactically: FAA guidance to operators gives an onset of 15 to 30 minutes and an effect lasting 3 to 5 hours for a dose of about 200 mg, but taken late it disturbs the following sleep, and it masks fatigue rather than curing it.

Sleep disorders

Insomnia is difficulty sleeping. EASA texts divide it into clinical insomnia, an inability to sleep when the body is calling for it, usually a symptom of another problem, and situational insomnia, caused by disrupted work and rest patterns or time-zone changes, which is the kind most reported by aircrew.

Narcolepsy is an inability to stop falling asleep, even when the person is not short of sleep, and may strike at any time; it is incompatible with flying.

Sleep apnoea is a stoppage of breathing during sleep. Brief pauses are common, but when they are frequent and long the repeated awakenings fragment sleep and cause excessive daytime sleepiness. The FAA's AIM calls obstructive sleep apnoea (OSA) an important preventable factor in transportation accidents and lists its symptoms: snoring, excessive daytime sleepiness, breathing pauses during sleep, memory impairment and lack of concentration. It advises evaluation by a sleep specialist for anyone with those symptoms, a neck size over 17 inches in men or 16 inches in women, or a body mass index above 30, and notes that most treatments are acceptable for medical certification once shown to be effective. Under EASA Part-MED (MED.A.020), a licence holder aware of any decrease in medical fitness must not exercise the privileges of the licence and must seek aeromedical advice.

Sleep hygiene and sleep inertia

Sleep hygiene is the set of habits that make good sleep more likely: a regular routine where rosters allow, a cool, dark and quiet room, and no heavy meal, alcohol or caffeine near bedtime. EASA texts note that caffeine disturbs both deep (stage 4) and REM sleep, and that alcohol, although it brings on sleep, considerably reduces REM sleep and causes early waking. Barbiturates and benzodiazepines have no place in aviation except under the strict supervision of an aviation medical specialist, and melatonin also needs medical advice (see alcohol, drugs and medication).

Sleep inertia is the grogginess and impaired performance that follow waking, and it is worst after waking from deep sleep. Naps are planned around it. A short nap of about 10 to 20 minutes stays in light sleep and refreshes without much inertia; EASA texts give 10 minutes as the minimum for a nap to be restorative. Deep sleep begins after roughly 30 minutes, so a longer nap is better planned as a full cycle of about 90 minutes, and recovery time must be allowed before flying. Controlled rest on the flight deck, where permitted, applies the same logic with fixed limits (see fatigue).

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.