Sleep is actually vital for human survival. The exact mechanism or reason behind needing sleep is unknown but one of the more prominent theory is that sleep 'resets' the imbalance in neurotransmitter levels built up in the brain due daily activities.

In fact, there are very rare cases of people with lesions in their thalamus causing them to lose their sleep drive and therefore, irrevocably deprive them of their ability to fall asleep. This condition is known as familial fatal insomnia and is invariably fatal. Death usually occurs within 1-2 years and is preceeded by absolutely horrific symptoms, beginning with insomnia, fatigue, hallucinations, rapid weight loss, severe dementia, depressed consciousness and before finally falling asleep forever.

http://www-personal.umd.umich.edu/~jcthoma...s/AAkroush.html