The importance of sleep

On the face of it, sleep is an incredibly perplexing phenomena. We humans spend about a third of our lives asleep and when we’re sleeping we are entirely defenceless and passive. So you have to think sleep must be very important for our survival and our viability. Otherwise evolution would have weeded it out long ago. In fact, all animals sleep in some way or another. Even the humble fruit fly, Drosophila melanogaster, goes through stages of sleep. In fact, studying Drosophila has provided many important clues on the molecular and genetic basis of sleep in other animals, even in humans. And sleep is also very variable across the animal kingdom. Dolphins sleep with one half of their brain at a time.

Koala bears sleep for about 22 hours each day and giraffes get by on less than four. So sleep is not just a simple period of turning off during darkness. It’s far more than that. It’s a highly complex, highly regulated state of the brain and it’s a state of the brain in which our conscious experiences change, too. So let’s think about human sleep. And there are many different kinds of human sleep but we can broadly think of them in two categories. There’s so-called non-REM, non-rapid eye movement sleep, also called slow wave sleep. This is the only normal form of unconsciousness that human beings ever undergo.

We can lose consciousness in anaesthesia or after brain injury but the only time we lose consciousness in normal life is during these stages of slow wave or non-REM sleep. Then there’s REM sleep, REM standing for rapid eye movement. It’s this stage of sleep that is most commonly associated with dreams. But as we’ll see, dreams can happen in other stages of sleep, too. Digging a bit deeper into the different stages of sleep, we can think about what happens in the brain that characterises these different stages.