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Why do we sleep? What happens to us during sleep? What are the consequences of missing sleep? How can we obtain maximum benefit from sleep?
We spend one-third of our lives asleep, yet we are only scratching the surface of understanding it. Sleep is one of those subjects that when you start asking questions, any answers just lead to more questions. Sleep, to borrow an observation from Winston Churchill: ‘Is a riddle, wrapped in a mystery, inside an enigma.’
What, apart from the obvious loss of consciousness, happens during sleep? Even that simple question conceals the fact that sleep is not binary. There are several levels of sleep. As we are drifting off to sleep, and waking up, there is a ‘half-way’ state, the somnambulistic state. In some rare cases during that state some people ‘walk in their sleep’.
Less dramatically, sleep is classified into five stages, one of which is REM sleep and the other four are various levels of Non-REM sleep, where REM means rapid eye movement. Studies over many years indicate that REM sleep is associated with dreaming.
What happens during REM or stage five sleep? We can observe someone’s eyes moving rapidly behind their eyelids, their breathing shallows and becomes rapid. As you will have experienced, when we are dreaming sometimes we know we are dreaming, and other times, it feels very real. In the dream we have all our senses and motor control. Fortunately, in reality, we do not; the body goes in to a form of partial paralysis during dreaming. The limbs are paralysed and the senses are largely switched off, with the hands and feet apparently able to move without fine motor control. Perhaps sleep paralysis is an essential adaptation that evolved to prevent our movement inadvertently harming us during dreams.
Sleep paralysis can last for a short while after we awaken, causing alarm and distress. You may have experienced a mild version of it: you become conscious, and suddenly your hearing kicks in, as though someone had just turned up the ambient noise level. Or, you may have experienced the most unpleasant version: night terrors, where you are conscious but, alarmingly, unable to control your body, and feel under attack by some strange force (the description of which varies across cultures).
Most of our sleep time is not involved in dreaming. The first four stages of sleep are called non REM. In stage one non REM sleep, there is slow movement of the closed eyes, no muscle movement, and only partial awareness of what is going on. Many people experience this stage as they fall partially asleep in front of the television.
In stage two sleep, we are really asleep: breathing and heart slow, body temperature drops, and eye movement either ceases or is very slow.
Stage three sleep is characterised by muscles becoming very relaxed, breathing slows even further, and brain activity, as indicated by brain waves, slows down. If someone is woken from stage three sleep they are typically disorientated (indicating that they were fully unconscious).
Stage four sleep is deeper still. Brain activity slows down even more. People in stage four sleep are very difficult to awaken.
During the course of a nights’ sleep we move in a cyclical way through the stages, with the deepest sleep taking place in the first three hours. As the night progresses the sleep becomes progressively shallower until we awaken.
There are some interesting phenomena in dreaming. Many studies have shown that we can incorporate outside signals into our dreams. You may have experienced that. Typically some external noise appeared in your dream, and as you woke up from the dream you realised that it was from a source different from that in the dream. One often reported is that the wake up call from their clock was a fire-engine (or some similar noise) in their dream.
Some people can even direct their dreams. You may have experienced waking up from a pleasant dream, and have chosen to go back to sleep to continue the dream, and have done so. What does that tell us? That some part of us is conscious of our dreams, recalls some of our dreams, and can even influence our dreams.
Throughout history people have reported solving problems in their dreams. For instance, the sewing machine invention was completed after Elias Howe dreamed that he was being attacked by cannibals who had spears with holes in their tips. Dimitri Mendeleev created the periodic table in dream. Paul McCartney wrote the most covered song of all time,“Yesterday” in a dream. The list of dreamed inventions and solutions is long.
Dreaming can be useful, is that why we dream? Is there some kind of processing going on in the brain that can’t be conducted when we are awake? Is that why we sleep? If rest was all that we required to recuperate, we could simply sit motionless in a chair for a few hours, and give our bodies a chance to recover from its exertions.
There are many theories about why we sleep, why we need sleep, what goes on during sleep, but that is all they are: theories. We still don’t understand the nuances and intricacies of what is happening during sleep. We do know that within 36–48 hours of being deprived of sleep severe mental dysfunction appears, starting with, among other problems, peripheral hallucinations, and becomes progressively worse.
Many disorders are related to, exacerbated, or caused by chronic sleep deprivation. For instance, depression. However the causal direction is not clear: does depression cause sleep disturbance, thus making the depression worse? Or does sleep deprivation cause depression, thus making the sleep deprivation worse? Or can both causal directions be true?
The complexity of what is going on in sleep is a long way from being understood. We can say what factors impede sleep, and can encourage people to avoid those. Alcohol and drugs have a detrimental effect, as does lack of exercise.
Some researchers in the field of sleep have long since thought that the study of sleep, reaching an understanding of sleep, will help unlock other mysteries of the mind, such as consciousness, how anaesthetics work, and memory.
Here is a fascinating observation from cetology, the study of marine mammals, such as dolphins. Half of their brain (which is about 23% larger than ours), can sleep while the other stays awake. Even more interesting, if they learn something when one half is awake, they can access that memory when the other half is awake. That must be telling us something important about sleep, about memory, about consciousness, but what? It is one thing to recognise that something is a clue in our attempts to understand sleep, it is quite another to know what that clue means.
Sleep is mostly a mystery to us, one that will keep researchers in work for centuries.
Professor Nigel MacLennan runs the performance coaching practice PsyPerform.