During sleep, the brain is taken off-line. It would be weird to design a machine so that it had to be shut down for a third of its operational lifespan. You can use your phone while it’s recharging – why not your brain? Sleep must be doing something pretty fundamental for the brain’s functioning. It’s not as if the brain is always resting during sleep – while you’re dreaming, it’s often consuming as much energy as in the waking state.
Sleep is made up of a number of stages, which can be distinguished by the different patterns of electrical activity generated by the neurons. When you’re dreaming, the eyes show rapid movements under the eyelids, so it’s called Rapid Eye Movement (REM) sleep. The body is paralysed during REM to stop you acting out your dreams. In the rest of sleep (less imaginatively known as Non-REM or NREM sleep), there are successive phases of deeper sleep, the deepest of which is characterised by slow synchronised neural activity (so-called slow-wave sleep) when the wick is turned down and energy consumption is reduced. A run through all the stages takes around 90 minutes in adults, shorter in children. An adult will fit in several 90-minute cycles in their 6-8 hours sleep.
Sleep has two types of function in the brain. The first is restorative: to clean away the waste products created by neural activity in the day, to make and mend. Cleaning occurs during NREM sleep. The second function is to strengthen memories and embed newly learned skills, that is, to make them more stable and effective. Partly this is to consolidate changes made to connections during learning, which is probably done in REM sleep. Partly, it is to integrate new learning into the existing information in the brain’s neural networks, while extracting the most important themes from new learning. This is probably done in NREM sleep.
One of the ways the brain achieves consolidation is to replay the experiences of the day, re-enacting the same patterns of neural activity. For example, to ship knowledge out of episodic memory and into longer-term conceptual memory, the brain re-establishes activity in the hippocampus while the cortex gradually listens and changes its connections. Sleep, therefore, is when knowledge is moved around between brain structures.1
If you reactivate a memory to ship it out of hippocampus, you probably wouldn’t want to record that memory in hippocampus again. You’d end up with two copies of the same memory! Maybe that’s why we often don’t remember our dreams as new memories.2
Sleep isn’t optional. When you don’t have enough sleep, you need to sleep more later to catch up (to repay the ‘sleep debt’). When it is deprived of sleep, the brain works a bit like it’s drunk. Learning is impaired. The modulatory system is particularly impacted: concentration cannot be sustained for long periods, you’re less mentally flexible, and you are more impulsive. Social functioning is impaired, too: you can be grouchy, dogmatic, and less likely to accommodate others.
The need for and nature of sleep changes with age. REM sleep in the early years plays a role in building the brain, with the internally generated experiences probably helping to organise brain connections. Prenatally, almost all sleep is REM. The newborn sleeps 16 hours a day, with half of it REM. The six year old sleeps 11 hours a day 3 and REM drops to a quarter. For the sixteen year old, it’s 9 hours, with REM still at a quarter. By 66, it’s 6 hours and REM at 10%.
Sleep does seem like a design flaw – a function that’s necessary because the brain ended up using networks of neurons as its basic mechanism for learning. Like I told you, evolution’s solutions are not necessarily the best ones.
 Read some more about sleep and memory here Here’s some chat about remembering dreams  This will be the average amount you need at each age so that you don’t wake up tired / grumpy / cognitively impaired the next morning