Sleep and biological rhythms

Sleep is a fundamental physiological need — not "lost time" but an active regeneration process for body and brain. Sleep deprivation is one of the most frequent causes of human error in aviation.

Sleep stages

Normal sleep consists of several 90-minute cycles with four stages:

Non-REM sleep (NREM):

N1 — fall-asleep phase, light relaxation, weak brain activity.
N2 — light sleep, heart rate and body temperature drop.
N3 — deep sleep (slow-wave) — physical regeneration, immune system, growth hormones.

REM sleep (Rapid Eye Movement):

Rapid eye movements, intense dreams, brain activity almost like waking.
Memory consolidation and learning.
Muscle atonia prevents dream-enactment.

Per night about 4–6 cycles; as the night progresses less deep sleep and more REM.

Recommended duration: 7–9 hours for adults.

Circadian rhythm

The circadian rhythm is the body's internal 24-hour clock, driven by the suprachiasmatic nucleus of the hypothalamus.

Zeitgebers (timing cues):

Main signal: light (daylight → wakefulness, dark → sleep).
Secondary: meals, activity, social contact.

Hormones:

Melatonin — released in the evening with darkness → sleepiness.
Cortisol — peaks in the morning → wakefulness.

Physical variations:

Body temperature low at about 03–05 local.
Reaction performance low: 03–05 and partially 13–15 (post-lunch dip).
Reaction performance peaks: 10–12 and 17–19.

Pilot relevance: night and very early morning flights tax the pilot heavily; reaction time around these troughs is measurably slower.

Jet lag

Flights across several time zones desynchronise the internal clock from external time → jet lag.

Rule of thumb: adaptation takes about 1 day per time zone crossed.

Harder: eastward (day is shortened); easier: westward.

Symptoms:

Sleep disturbance,
Fatigue at the wrong time of day,
Reduced concentration and judgement,
Irritability,
Digestive trouble.

Countermeasures (less relevant for GA pilots, more for airline):

Pre-flight shift of sleep,
Sunlight/activity on arrival,
Melatonin (max 0.5 mg) helps falling asleep,
Caffeine timed for wakefulness.

Fatigue

Fatigue is a state of reduced physical and mental performance. Two types:

Acute fatigue — short-term:

After long flight, poor sleep, demanding manoeuvring,
Recoverable through sleep.

Chronic fatigue — long-term:

Built up over days/weeks of constant sleep deficit,
Sleep debt — cumulative, cannot be cleared in one night,
Most important cause of human error in the cockpit.

Symptoms of fatigue

Early signs:

Concentration drops,
Attention wanes,
Irritability / mild aggression,
Yawning, sleepiness.

Late signs:

Microsleeps — 5–30 seconds unconscious without noticing,
Tunnel vision,
Tasks forgotten or wrongly performed,
Increased risk acceptance.

Dangerous: fatigue impairs self-assessment — the tired pilot believes they are still fit.

Fatigue management for pilots

Pre-flight:

At least 7–8 h sleep before demanding flights,
Avoid sleep loss the previous day,
IMSAFE check (lesson Fitness to fly — IM SAFE (alt. IMSAFE) self-check).

In flight:

Breaks and meals at sensible times,
Hydration (water, no alcohol),
On signs of fatigue: use standby functions (autopilot if installed), repeat routine checks more often.

No-go (pilot's discretion):

With chronic fatigue or sleep debt > 4 h: postpone or cancel the flight.
Right and duty of the PIC per Part-NCO.GEN.105.

Sleep hygiene

Recommendations for better sleep:

Regular sleep schedule including weekends,
Bedroom dark, cool, quiet,
No screens 1 h before sleep (blue light suppresses melatonin),
No caffeine 6 h before sleep,
No alcohol 3 h before sleep (destroys REM),
Exercise during the day.