It's an age-old question, why do we sleep? It is an odd phenomenon, especially when you consider how vulnerable we are when we're sleeping. From an evolutionary perspective it doesn't really make a lot of sense! There have been countless theories over the years as to why we sleep. However, several news outlets have recently reported a new 'sleep theory' which concludes that we sleep simply to allow the brain to cleanse itself of toxins built up from a day of activity (thinking!).
Just like our bodies' Lymphatic System cleans waste from the by-products of exercise, the theory is that the brain also cleans itself during sleep, so we wake up with a clear head ready for normal waking activity.
What scientists have discovered is that the fluid-filled spaces between brain-cells swell from about 5% during awake state to more than 20% during sleep. This fluid is used to clean waste from the brain while we sleep. Brain-cells also shrink during sleep to help the fluid cleaning process.