Sleep and Fatigue — Page 210, Lesson 334

I want to walk you through the relationship between sleep, body temperature, and performance — because this is central to understanding fatigue in aviation. Let’s start with a key concept: zeitgebers. That’s a German word meaning "time givers." These are the external cues that synchronise our internal body clock to the 24-hour day — things like clocks, set meal times, and the natural light-dark cycle. If you isolate someone from all zeitgebers — no clocks, no fixed meal times, no way to detect light changes — their circadian rhythms will free run. That means they drift away from 24 hours and settle into a natural periodicity of about 25 hours. So instead of the typical 16 hours awake and 8 hours asleep, that person will extend their day to about 17 hours awake and still 8 hours asleep. That’s a 25-hour cycle. Now let’s talk about body temperature and its direct relationship with sleep. There is a clear link between our body temperature cycle and our sleep-wake cycle. At the time of lowest body temperature, we find it hardest to stay awake. We start to feel sleepy when the temperature is falling, and we are at our most wide awake when the temperature is rising. This relationship explains why, after crossing several time zones, you may have difficulty sleeping well for a few days — that’s one of the symptoms of jet lag. Body temperature follows a regular cycle throughout the day. The highest temperature occurs around 1700 hours — that’s 5 PM. The lowest temperature occurs around 0500 hours — 5 AM. At that low point, we are least efficient, and the desire for sleep is at its peak. Now, when it comes to timing planned sleep, the amount of time you’ve been awake is important for determining readiness for sleep, but there’s also a circadian rhythm of sleep itself. This means that at certain times of day, even someone who is severely sleep-deprived may have difficulty falling asleep. The timing of sleep — not the amount of time awake — is the critical factor in determining how long you will actually sleep. And as I just mentioned, sleep duration is linked to the body temperature cycle. Here’s the practical takeaway for aircrew: Sleep taken near the temperature peak or when the temperature is falling will be longer and more refreshing than sleep taken when body temperature is rising. If you try to sleep when your body temperature is on the rise, you will have considerably more difficulty getting to sleep, and if you do manage to fall asleep, you will usually awaken within a relatively short period of time. Finally, let’s look at time of day and performance. Beyond the circadian rhythms of temperature and basic physiological processes, there are rhythms for more complex behaviours. Performance on different tasks is affected by the time of day. Simple tasks that require little short-term memory input follow the pattern of body temperature — performance improves as temperature increases and declines as temperature decreases. But short-term memory tasks — those decline throughout the day. Verbal reasoning and mental arithmetic skills peak around midday. And this has real-world consequences. Accident statistics have been examined to find a correlation between time of day and accidents. It has been found that driving accidents peak at certain times — a pattern that aligns with these circadian rhythms.