Alertness
According to the CAA Flight-crew human factors handbook CAP737,
Besides sleep, the other major influence on waking performance and alertness is the internal circadian clock. Circadian rhythms fluctuate on a regular cycle which lasts something over 24 hours when allowed to 'free run'. The circadian rhythms are controlled by the suprachiasmatic nucleus of the hypothalamus situated in the brain which is the time keeper for a wide range of human functions, including physiological performance, behavioural mood and sleepiness/alertness. Many body functions have their own circadian rhythm and they are synchronised to a 24 hour pattern by 'zeitgebers' (time givers). Light has been demonstrated as being the most powerful zeitgeber to synchronise the circadian pacemaker.
The mean normal body temperature is 37 degrees Celsius, but it has a natural cycle of less than one degree over the 24 hour period with a minimum at 06:00 hours, rising during the day to a maximum at 20:00 hours. In the normal rhythm, sleep would occur between 24:00 and 08:00 hours when body temperature is falling and reaching its low point. Therefore it is most difficult to work when body temperature is falling, and hardest to sleep when the body temperature is rising.
Moving to a new light/dark schedule (e.g. shift work or time zone changes) can create internal and external desynchronosis. This leads to a discrepancy between internal suprachiasmatic nucleus timing and external environmental cues. The internal clock can take days or weeks to readjust, but generally requires one day for each time zone crossed or one day for every 90 minutes of jet lag.
Crossing time zones is a way of life for long-haul flight crew, and constant time zone shifts can lead to cumulative sleep deprivation due to disruption of the body cycles known as circadian desynchronosis. This is also known as 'situational insomnia'. However, sleep debt and fatigue can also be problems for short-haul crew as a result of regular early morning starts and long multi-sector days.
Long-haul crew have to constantly adjust and readjust circadian rhythms, and the various intrinsic rhythms for temperature, digestion and excretion get out of phase with the rhythm for sleep. This leads to jet lag or circadian dysrhythmia.
Resynchronisation on westbound flights is aided by the body's normal circadian rhythm being nearer 25 hours, thus assisting the day to be 'stretched', whereas eastbound flights are more difficult due to the day being 'compressed'. Resynchronisation is easier when local time on landing is behind that at the airport of departure, whereas it is difficult when local time is ahead.
Factors affecting sleepiness include:
- Prior sleep and wakefulness;
- Circadian phase leading to –
- Increased sleepiness in the early hours of the morning and during the afternoon;
- Decreased performance in the early hours of the morning.
- Age (the amount of nocturnal sleep required reduces after the age of 50);
- Alcohol (reduces the quality of sleep);
- Work and environmental conditions.
It is vital that pilots and crew-members identify any sleep threat early on, because once sleepiness takes hold it can go unnoticed until too late. The following two anecdotes are not unusual.
- An ex-military pilot recounted that he had been on task in a prolonged auto hover at 40ft over the sea. It was the early hours of the morning when suddenly he awoke with a startle. He quickly looked around to see if anyone had noticed his slumber and to his horror he realised that the entire crew were sound asleep.
- It was late at night and the crew were on a prolonged surveillance task which involved hovering at altitude over a city. The commander recalled that after some time on task he had a sensation of falling and realised he had fallen asleep momentarily and lost control. In his startled state he stared at the attitude indicator but could make no sense of it or any of the other instruments to affect a recovery. This disorientation probably only lasted for a second or so but it felt like an age. Once control was regained it took more concentration than normal to maintain. The remainder of the crew, who had been focussed on the surveillance task throughout, were oblivious to the situation and calmly asked if they had encountered some turbulence.
