SLEEP – a thumbnail overview
Many people view sleep as a rather mysterious way of eliminating sleepiness. Not so much the elixir of life perhaps, but maybe it removes a few wrinkles and those ‘bags under the eyes’. If only there was a pill to abolish both sleep and sleepiness, then we could add half as much again to our waking lives. No more time-wasting in preparing for bed each night, save money on a smaller house without bedrooms, and all that free time for entertainment, socializing and surfing on the internet. No need to lose sleep over insomnia, no more sleep-related road crashes, freedom to fly at night without noise restrictions, and all that extra time with our ever-active kids who won’t be sleeping either. Ah, there’s a thought – maybe not such a such a sweet dream after all, but more of a nightmare, as a sleepless world would be a relentless one, everyone on the go, working round the clock and with little time to chill-out by oneself.
Sleep is not like the bedside light – a simple click, and off to oblivion. It is such an intricate process, involving so many parts and substances within the brain, that to do away with it would require a hitherto incomprehensible pot-pourri of drugs, which is why sleep is so intriguing. Besides, it dominates our lives, is far from being a soporific subject, and from what little we still know puts it well beyond the fantasy of dreams.
Even the simplest of all living organisms have regular, daily rest periods, overseen by an internal ‘body clock’. But when does rest become sleep ? Bees, scorpions and squid certainly sleep as, for them, it brings unconsciousness. What of plants at night, when photosynthesis ceases – are they unconscious? For humans and fellow mammals, the role of sleep will differ, depending on body size and complexity of the brain – from being an energy conserving immobilizer for the mouse, to the sole provider of rest for the higher centers of the human brain.
Do we grow in our sleep ?’ No – it’s mostly a matter of shrinking during wakefulness. Beauty sleep, disappearing wrinkles, ‘night starvation’ are based more on fiction than fact.
So why do we sleep ? It’s for the brain – our brain only comprises about 2-3% of our body weight, but it is relatively huge – not so much in size perhaps, but enormous in its demands. When awake it consumes a fifth of both the oxygen we breathe and calories we eat. Unlike all other organs, able to rest when we relax in front of the TV, the ‘human’ parts of our brain cannot rest like this, but only during sleep.
Interestingly, human adult sleep resembles that of an infant to a much greater extent than is the case with other mammals. For example, an adult dog’s sleep is much less like that of a puppy. In fact, our adult behaviour is more ‘infant like’ than is the case with other mammals. Not only do we retain curiosity and playfulness but more importantly, we are able to learn new things right up to old age. Whereas you literally ‘can’t teach an old dog new tricks’, this is not the case with us. So, how does all this relate to our sleep ? For all mammals, infant sleep is a time when its developing brain deals with all the new stimulation of wakefulness and learns how to deal with it all – we call it ‘brain plasticity’, which allows us to learn new things and, change our behaviour. Sleep helps with this process, but as plasticity ‘solidifies’ as the mammal becomes an adult, sleep doesn’t have so much of a role in this respect – but for us ! Plasticity does slow up somewhat as we get older, but this process never ceases, and that is why our adult sleep is so important in this respect, and is why we ‘sleep like a baby’.
Very severe sleep loss impairs this ‘plasticity’ and we become more like automatons working on ‘autopilot. Worse still is the profound sleepiness, of course. Our thought processes become impaired, and even one night without sleep will begin to affect the sleepy doctor, manager, politician, and soldier in being able to make decisions – can they really cope with emergencies ?
Nevertheless, some reduction in sleep can be therapeutic for those people with severe depression, whereas too much sleep can bring other problems. It is rather like eating – we can’t do without food; many of us would be healthier by eating a little less and losing weight, and we all know what happens if we continue to eat too much!
Literally, the greatest impact of sleepiness is from falling asleep at the wheel. The body’s natural 24 hour clock plays a crucial role, as not only we are most likely to be sleepy when we stay up in the small hours, but again in the early afternoon. On the other hand, we are least likely to be sleepy in the late afternoon and early evening. Called the ‘circadian rhythm’, its daily highs and lows are reflected both in body and behaviour, although we do differ somewhat. On the one hand there is the ‘lark’ – the early riser and life and soul of the breakfast table, who’s off to bed by 10 pm. Opposite is the owl – the late riser who doesn’t really get going until the evening. By the way, the afternoon dip is why even small amounts of alcohol around lunch-time make us so sleepy in the afternoon compared with early evening – alcohol itself has a soporific effect which will combine with the dip to make it much worse !
Types of Sleep
Given that sleep is by the brain for the brain, then the best ‘scientific’ way to measure it is by recording the minute electrical activity of the brain (the electroencephalogram – EEG) using small, silver discs attached to the scalp. This EEG is used to divide sleep up into stages, with the most characteristic being Rapid Eye Movement (REM) sleep having its eponymous ‘rapid eye movements’, when much of dreaming occurs, usually every 90 minutes during sleep and for about 20-30 minutes at a time. ‘non-REM sleep’ is divided up into stages 1 to 4, of increasing depth. Stages 3 and 4 are often combined as ‘slow wave sleep’and called ‘deep sleep’. As seen in the hypnogram, these sleep stages cycle throughout the night. Soon after sleep onset there is usually a rapid descent to stages 3 and 4 sleep, which reappears less so the second cycle and only somewhat in the third cycle. The first period of REM sleep is usually a little shorter than the rest. In sum, slow wave sleep i concentrated more in the first half of sleep and REM in the second half.
For me, the enigma of REM sleep comes not from the dreams themselves, nor with the body’s paralysis that accompanies REM sleep, but whether it is sleep at all. Is it a type of wakefulness, or ‘non-wakefulness’, like a computer in ‘screen saver’ mode? The period of our lives when REM sleep is most abundant is during the month or so before birth. And here is where its real purpose may lie – in stimulating the developing brain. Not only are we physically paralysed during REM sleep but our emotions are switched off as well. Nevertheless, we can live quite happily without REM sleep, and I suspect that dreams are the ‘cinema of the mind’ – merely best-forgotten B movies, with the fantasizing being in the dream interpretation rather than with the dreamer.
How much do we need ?
Findings in the UK over the last 40 years consistently show the average daily sleep for adults is about 7¼ hours, which is no less than it was a hundred or so years ago. Mortality seems to be lowest in those sleeping around 7 hours a night. Nevertheless, there are claims that we should be sleeping for longer (8-9 hours), and that habitually short sleep (‘sleep debt’) also leads to physical problems such as obesity, the metabolic syndrome (‘glucose intolerance’), and maybe even diabetes. I disagree that ‘sleep debt’ is worse today than it ever was, or that it really is a risk factor for obesity, etc. Often, small statistically significant findings are wrongly assumed to be of medical importance, when they are not. Sleep, like food and drink, can be taken to excess of biological needs.
Zoo animals sleep more than in the wild, horses longer in stables than in fields, and the well fed cat more than the one that has to hunt for its supper. We also can sleep well beyond our usual levels if we stay in bed all day, but we don’t need it – just like that extra helping of yummy dessert and all those cans of Coke. These are not for satiating hunger or thirst but for pleasure – and the same goes for some of sleep. So, how much of our usual sleep is really necessary? After a night of no sleep, the next night we will regain less than half of that lost sleep, and feel fine. We regain all of the lost “deep” (stages 3 and 4) sleep, which seems to be the sleep most beneficial to the slumbering cortex.
There remain many inexplicable aspects to sleep. One is that REM sleep can be removed entirely from our sleep, for many months at a time, by medicines commonly used for treating depression, and without ill effects or any sign of big rebounds of REM sleep when treatment ceases. This seems to negate the belief that REM sleep is essential to memory processing, especially as dolphins have no REM sleep whatsoever. In fact, and in many respects of REM sleep may not be ‘real sleep’, as is non-REM sleep, but can be seen more as “non-wakefulness” – even as a substitute for wakefulness, particularly in the baby before birth and for a few months afterwards.