Body Rhythms and Sleep

A pilot's most important flight instrument may be the one issued at birth: the internal clock that schedules sleep, body temperature and alertness around a daily cycle. This lesson reads that clock, takes a night of sleep apart into its stages, shows how to budget sleep like fuel, and then does what aviation does to every clock it meets: flies it across nine time zones and asks it to cope.

The body's internal rhythms

Many body functions rise and fall rhythmically whether their owner is awake or asleep, under internal control routed through the hypothalamus. The rhythms the syllabus names: body temperature, heart rate, blood pressure, sensory acuity, adrenal hormone output and brain neurotransmission. For sleep, one of these outranks the rest: body temperature is the most important rhythm, because the ease of falling asleep, the length of the sleep and its quality all track where the temperature cycle stands.

Rhythms with a period of about a day are circadian rhythms (circa dies, about a day). Left to itself the cycle is not exactly 24 hours: a person isolated from every time cue, with no clocks, daylight or set meals, settles into a free-running rhythm of about 25 hours, drifting later each day. In normal life the clock is trimmed back to 24 hours by external time cues called zeitgebers: sunrise and darkness, clock time, meal times, work schedules and social routine. Light is the most powerful of them — morning light nudges the clock earlier (a phase advance), evening light stretches it later (a phase delay), which is how the 25-hour clock is held to the 24-hour planet.

Body temperature, performance and the low point

Core temperature follows a fixed daily curve: highest around 17:00, lowest around 05:00. Performance on simple, routine tasks follows the same curve, best when temperature is high and worst around its trough. The hours around the minimum, roughly 02:00 to 06:00 on the body's own time, form the window of circadian low: the period when sleepiness is deepest, mental and physical performance are poorest, and a disproportionate share of fatigue accidents accumulate. A second, shallower dip arrives in mid-afternoon — the post-lunch dip, which occurs whether or not lunch is eaten.

The temperature curve also governs sleep itself. Sleep comes easiest when temperature is falling and normally begins about five hours before the minimum; waking follows about three hours after it. Sleep attempted while temperature is rising is the losing move: falling asleep is difficult and the sleep that does come is cut short, which is why the day sleeps of jet lag and night duty feel so thin. Timing decides duration — when you sleep matters more than how tired you are.

The architecture of sleep

Sleep laboratories watch three signals: brain activity on the EEG, eye movement on the EOG and muscle tone on the EMG. They reveal two different kinds of sleep alternating through the night. Non-REM sleep deepens through four stages: stage 1 is a transitional doze of about ten minutes; stage 2 is light sleep that ends up filling about half the night; stages 3 and 4 are slow-wave sleep, the deep, hard-to-wake sleep of large slow EEG waves, also called orthodox sleep.

Riding over those stages comes REM sleep, rapid eye movement sleep, also called paradoxical sleep because the EEG looks awake while the sleeper plainly is not. The eyes dart behind closed lids, heart rate and breathing turn irregular, vivid dreams run, and the body's muscles are held in near-total paralysis so the dreams cannot be acted out. Non-REM is a quiet brain in a quiet body; REM is a busy brain in a paralysed one. Over a full night roughly three quarters of sleep is non-REM and one quarter REM.

The two kinds alternate in a cycle of about 90 minutes. The first descent reaches slow-wave sleep quickly, and about 90 minutes in the first REM period arrives, lasting only 10 to 20 minutes. As the cycles repeat the balance shifts: slow-wave sleep dominates the first cycles and fades away; REM periods lengthen toward morning. A normal waking rises out of a long final REM period, dream in hand.

What sleep is for

The two kinds of sleep do two different jobs. Slow-wave sleep restores the body: tissue repair and physical recovery, and a day of heavy physical work buys more of it. REM sleep restores the brain: it strengthens and organises memory, and a day spent learning new procedures buys more REM. REM also underwrites emotional balance, which is why disrupted sleep so reliably produces irritability. Deprive a sleeper of either kind and the next night shows a rebound: the missing kind arrives earlier and lasts longer until the debt is repaid, with slow-wave sleep taking priority.

Run short of sleep and everything degrades together: attention and vigilance decay, reactions slow, judgement and decision-making coarsen, mood sours, and microsleeps appear: involuntary lapses of a fraction of a second to a few seconds that the pilot is typically unaware of, which restore nothing. Losing even a few hours a night produces significant impairment within two or three days, the deficit is cumulative, and its effects bite harder at altitude. The only remedy is sleep: recovery from a real debt takes a minimum of two full consecutive nights, and no amount of training, motivation or caffeine substitutes. Naps genuinely help (ten minutes or more restores measurable alertness), but waking brings sleep inertia, a groggy, slow-reacting period that is worse after longer naps and can last twenty minutes or more, so a nap needs a planned margin before it can be trusted with an approach.

Sleep credit and sleep debt

The sleep budget can be run as a simple account: every hour asleep earns two points of credit; every hour awake spends one. A full eight-hour night earns 16 points, and 16 points is the ceiling — sleep cannot be banked in advance, so a twelve-hour lie-in before a long duty still buys only 16 hours of quality wakefulness. When spending outruns earning, the shortfall carries forward as cumulative sleep debt, the account behind the two-or-three-day decline above.

Three planning rules turn the account into a rostered sleep plan:

One hour of sleep buys two hours awake — so an expected 16-hour duty day needs eight hours of sleep behind it.

Take the required sleep immediately before the wake-up call for duty, so none of the credit is spent before the duty begins.

The three-in-one rule: divide the hours until the main sleep must begin by three to find the sleep needed to bridge them — 16 hours to fill costs 16 ÷ 3, about 5 hours 20 minutes of sleep now.

Crossing time zones

Fly the 25-hour clock across meridians and it arrives set to the wrong town. The result is circadian disrhythmia, jet lag: sleep that will not come at local night, wakefulness that collapses at local noon, disturbed digestion and appetite, degraded performance and a flattened mood. Resynchronisation is slow — the clock shifts about 90 minutes per day in the new zone, so the nine-hour shift of a Europe-to-US-West-Coast flight needs about six days, longer than almost any layover. Worse, different body systems shift at different rates, so during adjustment the rhythms are out of step with each other as well as with local time.

Direction matters. Flying westward lengthens the day, and a clock that free-runs to 25 hours lengthens gladly: London to New York, five zones west, makes a 29-hour day and leaves about four hours of lag. Flying eastward shortens the day to 19 hours against the clock's preference, leaving about six hours of lag from the same five zones. Westbound is easier than eastbound — the exam's standing conclusion, and every long-haul body's lived one.

The duty day and the coping strategies

Across a duty, fatigue is set by three clocks running at once: time since the last proper sleep, time on task, and the circadian time of day. Vigilance sags as hours on duty accumulate, and when the duty's later hours coincide with the window of circadian low the two decays multiply — the last approach of a night rotation is flown by a pilot at the bottom of both curves. On long-haul the temperature minimum can arrive in flight, mid-ocean, and a crew that knows its own clock can predict that trough and plan rest and in-seat naps against it rather than be ambushed by it.

The strategies that actually work, for the exam and the roster alike:

Short stopover: stay on home time — for a one-to-two-day layover, do not try to adapt; eat and sleep on home-base hours and target the body's own sleepy windows.

Longer stay: adapt deliberately — shift to local routine at once, use daylight exposure, local meal times and activity as zeitgebers, and avoid long daytime sleeps (limit any to two or three hours) so local night sleep can form.

Protect the sleep you can get: dark, quiet, cool room around 21 °C, a wind-down routine, and no caffeine within four to six hours, heavy exercise, large meals or more than minimal alcohol before bed.

Nap tactically — before night duty and against a predicted in-flight trough, with the sleep-inertia margin planned in.

Treat pharmacy as a last resort: melatonin is unregulated in quality, wrongly timed it deepens the problem, and sleeping tablets belong only under aeromedical advice after a ground trial — never with alcohol.