We sleep for a third of our lives, but how do we slumber and what is the function of sleep? Victoria Lambert asks the experts
With our electric blankets, pocket-sprung mattresses and draft-proof bedrooms, we sleep better in the 21st century than ever before. Yet to hear us talk, it would seem no one ever gets enough shut-eye.
“The more people worry about sleep, the more they worry and so sleep less,” says Jim Horne, professor of psychophysiology at the University of Loughborough, who is one of the UK’s leading sleep experts and author of Sleepfaring. “Yet our sleeping circumstances are better than ever, and we understand more and more about the nature and purpose of sleeping and dreaming.”
Recent research has challenged theories about why we sleep. For example, research from the Centre for Translational Neuromedicine at the University of Rochester Medical Center in New York has demonstrated for the first time that the space between brain cells may increase during sleep, allowing the brain to flush out toxins that build up during waking hours.
“Sleep changes the cellular structure of the brain,” says Dr Maiken Nedergaard, a leader of the study, which was published last October in the journal Science. “We need sleep – it cleans up the brain.”
It is clear that the night-time molecular processing – a sort of brain housekeeping shift – benefits our cognitive ability as well as wellbeing so that we wake refreshed in both cell and spirit.
But how do we sleep? What happens when we close our eyes and drift away to the Land of Nod? Traditionally, says Professor Horne, sleep has been analysed by charting it in four stages, which cover the changes in our brainwaves, measurable by EEG – electroencephalography, the technique that records the brain’s electrical activity – and including time when we are in REM (rapid eye movement) sleep and in non-REM (NREM) sleep.
Growth hormone is released during NREM, encouraging muscle and tissue repair, and stimulating the immune system to defend against infections. REM is a period for the brain to restore, cleaning out all the “junk mail” that has accumulated, and allowing the capture of memories and learning.
The first stage is drowsy sleep. Our muscles are active and eyes roll slowly, opening and closing moderately. Number two is “deeper sleep”, when we may twitch slightly or move about, but we are definitely asleep; a stage which can take up almost half our sleep time.
Stages three and four follow; we are in deep sleep now and would be hard to wake. Throughout the night, these stages are interrupted by 90-minute periods of REM sleep, when we dream vividly, twitch and may even wake briefly. This is the deepest period of sleep when blood pressure, heart rate and respiration lowers, and has been referred to as “human hibernation”.
If you have unhappy thoughts during the day, you’ll have unhappy dreams; happy thoughts during the day will see you happy in your dreams
Sleep clinics have used these different stages for clinical and diagnostic purposes to help those with insomnia or other sleeping problems. But Professor Horne believes a fresh approach is needed that regards stages three and four as one phase of sleep. He feels that a better way to classify sleep is to examine the brainwave patterns.
The fluctuations in activity can trace the brain moving between states of wakefulness, like drowsiness or REM sleep, and periods of deep NREM sleep. “To get into sleep, we must switch off wakefulness,” he explains.
“In REM sleep, unlike NREM, the brain listens to what is going on around, and the sleeper can still decide whether any sounds are meaningless, to be ignored, even incorporated into an ongoing dream, or if ‘emotive’ then there is an instant awakening, with the sleeper alert and ready to react. In this latter respect, REM sleep is like ‘screensaver’ mode on your computer – able to switch into immediate action.”
He adds: “In NREM sleep, your brain has been switched off properly, whether that is in stage two or three/four. To wake up, your brain has to reboot, which is why it takes longer to ‘come to’ from NREM than REM periods.”
NREM is probably a period of organisation for the brain. Professor Horne believes it is not just for consolidation of memories, but retuning and redirecting links. He is cautious about the new US research suggesting it’s a time for toxins to be flushed away. “Inevitably some chemicals will be drained away naturally, but new ones will appear too. It’s more complex than just a deep-cleaning mode,” he says.
But it is the purpose of REM sleep that perhaps looks most interesting for future study. We tend to think of this period as dream sleep but, explains Professor Horne, even in NREM sleep the brain is thinking about things. However, we never remember any of this and only remember dreams if we wake swiftly after them, which is impossible from deep sleep, and then we can only recall the last minute or so of the dream. In fact, the actual process of dreaming is facilitated by a different part of the brain to that of REM sleep itself.
It seems the purpose of REM is about stimulation, keeping our brains ticking over when there is no other stimulus. Babies enjoy REM sleep for well over eight hours a day in the womb. Once they hit the real world, full of new stuff, their need for REM sleep drops quickly, falling to about five hours a day at the end of the first year. Incidentally, sea lions don’t have REM while they are at sea and only experience it on land.
REM, periods of which get longer and more intense as we approach morning, seems to prepare us for waking up after a period of what Professor Horne has called “core sleep”, the first five to six hours of mostly NREM sleep.
One theory concerning REM is that we use this time to process and arrange our thoughts from the day, hence the bizarre dreams which can include aunts, uncles, unicorns and cabbage.
Professor Horne doesn’t believe that REM and dreaming sort out your psyche: “If you have unhappy thoughts during the day, you’ll have unhappy dreams; happy thoughts during the day will see you happy in your dreams.”
He concludes: “A good night’s sleep comes with peace of mind.”