Night 1. Most people are capable of going without sleep for a night. The experience is tolerable if uncomfortable.
Night 2. The urge to sleep is much stronger, particularly between 3-5 a.m., when the body temperature is at its lowest.
Night 3. Tasks requiring sustained attention and mental calculations become seriously impaired. This is particularly the case if the task is repetitious and boring. Again, the early hours are the most crucial to needing sleep.
Night 4. From this night onwards, periods of micro-sleep occur. People stop what they are doing and stare into space for up to three seconds. The end of micro-sleep is accompanied by a return to full awareness. Confusion, irritability, misperception and the 'hat phenomenon' occur. In this, a tightening around the head is felt as though a hat too small for the head is being worn.
Night 5. On top of the effects previously mentioned, delusions (false beliefs) may be experienced. Intellectual and problem-solving abilities are largely unimpaired.
Night 6. Symptoms of depersonalization occur and a clear sense of identity is lost. This is called sleep deprivation psychosis.
Adapted from Huber-Weidman, 1976.