Sleep and Fatigue — Page 216, Lesson 340

I want to walk you through the characteristics of orthodox and paradoxical sleep, how they cycle through the night, and what happens when we miss some of either type. Let’s start with the two main kinds of sleep: orthodox sleep and paradoxical sleep. These are the technical names. Orthodox sleep is also called slow‑wave sleep, and paradoxical sleep is what we know as REM sleep – rapid eye movement sleep. The book gives us a table of characteristics that contrast them directly. First, the EEG – that’s the electroencephalogram, which records brain waves. In orthodox sleep, the brain produces large, slow waves. In paradoxical sleep, the brain waves are high frequency – much faster and more like an awake pattern. Next, the EOG – the electrooculogram, which tracks eye movements. During orthodox sleep, the eyes are still. During paradoxical sleep, you get rapid eye movements – that’s where the name REM comes from. The EMG – electromyogram, which measures muscle activity in the throat – shows relaxed muscles in orthodox sleep, but tensed muscles in paradoxical sleep. That tensing is part of the body’s mechanism to prevent you from acting out your dreams. The ECG – electrocardiogram, the heart rhythm – is regular during orthodox sleep, but irregular during paradoxical sleep. Dreaming: in orthodox sleep, you normally have no recall of dreams. In paradoxical sleep, you do recall them – that’s when vivid dreaming occurs. Sleep walking: it can happen during orthodox sleep, but never during paradoxical sleep. Body movements are less frequent in orthodox sleep, more frequent in paradoxical sleep. Stomach acids are steady during orthodox sleep, but they increase during paradoxical sleep. And the function – the purpose – of each type is different. Orthodox sleep is for tissue restoration – physical repair and recovery. Paradoxical sleep is for memory organization – processing and consolidating what you’ve learned. Now, how do these two types fit together across a night? The book describes sleep cycles. During any normal night’s sleep, the pattern operates on an approximately 90‑minute cycle. Towards the end of the first 90 minutes after falling asleep, the first REM stage occurs. That first REM experience lasts only 10 to 20 minutes before the person passes back into slow‑wave sleep – back into orthodox sleep. At the end of the second 90‑minute cycle, the duration of REM sleep periods increases – so REM gets longer as the night goes on. The book refers to a sleep profile shown in Figure 11.3, and it tells us that the individual stages will vary depending on the activities prior to sleep. If a great deal of strenuous physical activity has taken place, then sleep stages 3 and 4 – those are the deepest stages of orthodox, slow‑wave sleep – will be extended. Alternatively, if a lot of mental work has been undertaken, such as learning new information or procedures, then REM sleep will be increased. So your body prioritises the type of sleep it needs based on what you did during the day. Now, what happens if you miss some of one type? That’s the rebound effect. Sleep deprivation experiments have shown that if a person is deprived of either slow‑wave sleep or REM sleep, there will be a rebound effect in the next sleep period. That means the individual will make up the deficit in either case. For example, if you are woken after 3 hours of a normal sleep period, your body will have had all its required slow‑wave sleep – because slow‑wave sleep dominates the early part of the night – but you will be deficient in REM sleep. In the next sleep period, it is found that REM sleep will occur earlier and last longer than normal. The body tries to catch up on what it missed. Finally, the book touches on age and sleep. Individuals differ in the amount of sleep they require. In a survey of one million people, the most frequently reported sleep duration was between 8 and 9 hours. Some people seem able to do with much less sleep and can manage quite well on 3 to 4 hours per night. Ageing brings major changes in sleep requirements. New‑born babies may sleep for up to 23 hours per day, of which the majority is REM sleep. Even as they grow older, they will require much – the excerpt cuts off there, but the key point is that sleep needs change dramatically across a lifetime, with REM dominating in infancy.