If you are looking for a serious answer (?)
sleep is necessary for the mind to sift through the sensory input it has received while in a conscious state, and decide how to process it. This takes time, and the mind needs to be in a fugue state to carry out this activity, with sensory input reduced to a minimum. Failure to allow the mind time to carry out this process will result in psychosis, and death.
Time to die without food and water - approx. eleven days.
Time to die without sleep - approx. five days.
Ask any parent of a new baby why sleep deprivation is such an effective torture!