Lack of sleep in the long run will cause death. There is one family that is afflicted with this hereditary disorder.
Sleep is more than rest for the next day. It allows your body time to repair and heal. Sleeping less than 6 hours a day is associated with increased greatly increased risk of cancers (no time to heal or repair) cardiac and cerebral accidents.

Driving on less than 6 hours sleep is equivalent to being legally impaired with alcohol, you are not operating properly, your brain is not totally engaged.