Here's why people need to sleep:
The brain does its "cleaning up" during sleep. As cells in the nervous system are active, waste products are produced. In the rest of the body, waste removal is carried out by the lymphatic system, but this system does not extend to the brain. Instead, cerebrospinal fluid (CSF) must be pumped through the brain tissue in order to flush waste back into the circulatory system where it can be excreted by the liver. Pumping CSF seems to require a great deal of energy, and researchers speculate that this energy expenditure is incompatible with the intense metabolic demands of brain cells during wakefulness. Also, cells in the brain shrink considerably during sleep, resulting in a 60% increase in interstitial space which allows CSF to wash more freely through the brain tissue. The consequence is significantly faster clearance of neurotoxic waste (e.g., β-amyloid).