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For as long as human beings have been able to reflect on anything, sleep has been one of the most impenetrable mysteries. The average person will spend three whole decades of their life asleep, and not sleeping is eventually fatal.
We know it must be important because all animals sleep. In fact, sleep is so fundamental to animal life, that you don’t even need a brain to do it.
If we go back 550 million years ago, to look at the ancestor of all vertebrate life, an aquatic flatworm very similar to the creatures today we call lancelets or amphioxi, it had an eyespot, a mouth, a digestive system and rudimentary nervous system but no central brain. Even so, its simple metabolic system ran to a circadian rhythm and produced a cycle of sleeping and waking.
And even if they don’t ‘sleep’ in the same way as animals do, plants, fungi and even bacteria also operate according to a 24-hour circadian cycle.
Sleep, it seems, is as fundamental to our survival as respiration, nutrition and reproduction.
But because it leaves us immobile and apparently unaware of our surroundings, it’s harder for us to see what evolutionary advantages it confers. But we are beginning to unravel its mysteries.
The use of increasingly sophisticated MRI brain scanners means we now know there are four stages of sleep that run on a 90-minute repeating cycle.
The first three stages are non-rapid eye movement or NREM sleep. Each of these gets deeper, so that by the time we reach stage three, the sleeper is very difficult to wake. REM sleep is the last to appear in the cycle and, on a good night, we might expect four of five REM sleep stages, each one lasting longer than the last.
NREM sleep appears to have two functions: memory transfer and cleaning. Our daily activities generate memories which are encoded temporarily in our hippocampus. During the slow, synchronised, waves of deep sleep these are moved from the hippocampus to the long-term storage sites in the brain’s cortex.
At the same time, the network of cells that make up the brain’s connective tissue are flooded with spinal fluid. During this process, these cells – known as glia – shrink by up to 60% to maximise the effects of the power wash. This flushes out the metabolic wastes and toxins that have accumulated over a busy day’s cerebral activity, in a similar way to the lymphatic system does for the rest of the body.
This combination of filing and cleaning makes intuitive sense, but what about the function of REM sleep – sometimes called ‘paradoxical sleep’ because the brainwaves it generates are impossible to distinguish from those of wakefulness?
In fact, the emotional centres such as the amygdala and the cingulate cortex are 30% more active during REM episodes than when we are awake. We know these must be performing some important evolutionary function because humans, on average, spend 20 to 25% of our sleeping time in REM, whereas the average for other mammals is 9%.
In his bestselling primer on the state of modern sleep research, Why We Sleep, Mathew Walker suggests that REM sleep and the dreams that characterise it are best seen as a process of integration, whereby the raw material of our daily experience combines with previous experiences in order to create “an ever more accurate model of how the world works, including innovative insights and problem-solving abilities”.
He points out that one of the many striking things about REM sleep is the suppression of the brain’s chemical for fight-or-flight response, noradrenaline. This creates a low-stress environment in which we can reprocess emotional memories, with one of two potential outcomes: either to recall the salient details of the useful ones that are worth storing, or to reduce the painful emotional charge that the others are carrying.
This might explain how we can separate emotion from experience in the memories we store, enabling us to recall traumatic events without having to relive their full emotional intensity.
Strong evidence to support this theory comes from the treatment of people suffering from post-traumatic stress disorder. One of the main symptoms of PTSD is intrusive and repetitive nightmares. By using drugs to lower the levels of noradrenaline, longer and more normal REM sleep can often be induced, leading to a sharp reduction in nightmares.
In her book The Committee of Sleep, Harvard psychologist Deirdre Barrett has a simpler theory. “Dreaming,” she writes, “is just thinking in a different biochemical state.” And the vexed question of dream interpretation, “just helps get in touch with our own metaphors a little more.”
One thing seems certain: to be able to think clearly and creatively about our lives and our collective future, we could all do with more sleep.