Why do we sleep?
“The only known function of sleep is to cure sleepiness,” the Harvard sleep scientist Dr J Allan Hobson once joked. This isn’t quite true, but the questions of why we spend about a third of our lives asleep and what goes on in our head during this time are far from being solved.
One big mystery is why sleep emerged as an evolutionary strategy. It must confer powerful benefits to balance out the substantial risks, such as being eaten or missing out on food while lying dormant. The emerging picture from research is that sleep is not a luxury but essential to both physical and mental health. But the complex and diverse functions of sleep are only just starting to be uncovered.
What’s going on in our brains while we sleep?
The brain doesn’t just switch off. It generates two main types of sleep: slow-wave sleep (deep sleep) – SWS – and rapid eye movement (dreaming), or REM.
About 80% of our sleeping is of the SWS variety, which is characterised by slow brain waves, relaxed muscles and slow, deep breathing. There is strong evidence that deep sleep is important for the consolidation of memories, with recent experiences being transferred to long-term storage. This doesn’t happen indiscriminately though – a clearout of the less relevant experiences of the preceding day also appears to take place. A study published last yearrevealed that the connections between neurons, known as synapses, shrink during sleep, resulting in the weakest connections being pruned away and those experiences forgotten.
Dreaming accounts for the other 20% of our sleeping time and the length of dreams can vary from a few seconds to closer to an hour. Dreams tend to last longer as the night progresses and most are quickly or immediately forgotten. During REM sleep, the brain is highly active, while the body’s muscles are paralysed and heart rate increases, and breathing can become erratic. Dreaming is also thought to play some role in learning and memory – after new experiences we tend to dream more. But it doesn’t seem crucial either: doctors found that one 33-year-old man who had little or no REM sleep due to a shrapnel injury in his brainstem had no significant memory problems.