Question: During sleep there are five distinct stages. Brain activity increases from stages 1-4, even though you become harder to wake. This on it's own requires an answer. However, what really puzzles me is the fifth stage (REM), in which brain activity almost completely ceases and your body becomes paralysed, with your eyes shaking rapidly (hence REM (rapid eye movement)). Why does this happen and what is actually going on inside your brain at this time?

Suzi Gage answered on 23 Jun 2011:

Hi @harrycohen1w and @charliepl10
Thanks for the question!
To answer part one – although brain activity is ‘increasing’, it’s a quite different activity to the brain activity when you’re awake. Scientists are not really sure what these brain patterns are doing, but you’re still getting deeper to sleep through these 4 stages, even though there is all this brain activity, so that’s why you get harder to wake.

As for Rapid Eye Movement…Well…I’m not aware of any research in to why the eye movement itself occurs. It’s thought that this is the time when you dream, and your body becomes paralysed so you don’t act out your dreams. It’s also thought that this is the ‘best’ kind of sleep in terms of rest and restoring your body’s mental functions ready for the next day, and it’s also the part of sleep where it’s hypothesized that memories are consolidated (reinforced), which is also one explanation for why we see people and things we recognise in our dreams.

I’m sorry that a lot of this is theory rather than evidence, but I hope you find it interesting, I found it fascinating, thanks for the great question!