You need sleep as much as you need food. Many people neglect the importance of having enough sleep without knowing that humans can actually survive longer without food than without sleep. With water alone, an average person could survive a month to two without food (that also varies from an individual’s body fat and other factors).
But a few sleepless nights will cause the person to start experiencing radical personality and psychological changes. The longest recorded time anyone has ever gone without sleep is 264 hours. At the end of the experiment the subject, Randy Gardner, did not stumble or hallucinate, but scientists warned that such a lengthy period without sleep is dangerous.