Sleep and Fatigue — Page 210, Lesson 331

I want to walk you through the topic of sleep and fatigue, starting with why sleep matters and how our internal body rhythms control it. Sleep is essential to human well-being. During a sleep period, your body isn't just recuperating from the physical activity of the day — it's also carrying out essential organization of your mental processes. The amount of sleep you need varies according to three factors: your age, the amount of physical and mental energy you used before sleeping, and individual differences between people. Sleep exhibits particular cycles during each sleep period, varying from light dozing to very deep sleep, with intervals of a unique type of sleep in which vivid dreams occur. The duration of sleep and its quality depend to a large extent on our internal body rhythms, so it's well to consider these rhythms before looking at sleep itself. Now, I want to emphasize something critical for you as a future aircrew member. You must not regard sleep as merely a mechanism for recuperation from the previous day's activity. It is of fundamental importance that your attitude towards sleep is proactive — you must actively plan your sleep so that flights are conducted at maximum physical and mental efficiency. This is not optional; it's a professional requirement. Let's move into biological rhythms and clocks. Many physiological processes in the body undergo rhythmic fluctuations, and these are controlled through the hypothalamus in the brain. These fluctuations occur whether you are awake or asleep. The rhythms are controlled internally — they are not simply a reaction to our environment. The rhythms are not necessarily synchronized with each other, and these fluctuations or cycles can vary from about 90 minutes to as much as 28 days. The discipline of studying these rhythms is called chronobiology. The most common rhythms exhibited by humans and most other animals have periodicities of about 24 hours, and these are known as circadian rhythms. The word 'circadian' comes from Latin — 'circa' meaning 'about' and 'diem' meaning 'day'. These rhythms are seen as measurable and regular daily fluctuations — sometimes greater than 50% of the daily mean — in variables such as body temperature, blood pressure, heart rate, sensory acuity, adrenal gland output, and brain neurotransmission levels. In normal conditions, our circadian rhythms are locked to 24 hours by external time cues. These cues are provided by clock times and other external events, such as the sun rising, light and darkness, the increase in traffic noise at certain times, regular mealtimes, and work schedules — all of which assist in the regulation of our internal biological clock. These cues are known as zeitgebers, which is German for 'time givers'. In fact, cognitive awareness of the clock time is, in itself, an example of a zeitgeber. So to summarise what we've covered: sleep is essential for both physical recuperation and mental organisation, aircrew must plan sleep proactively, our internal rhythms are controlled by the hypothalamus, the most important of these are the 24-hour circadian rhythms affecting body temperature, blood pressure, heart rate, sensory acuity, adrenal output, and brain neurotransmission, and these rhythms are locked to the external world by zeitgebers — time givers like clock time, light, noise, meals, and work schedules.