How does day and night cycle affect human health?

It is plain unhealthy to go for days without sleep. Your body simply is not capable of staying awake for too long. The world record for sleep deprivation is 11 days. Short term side effects of such sleep deprivation may be short term memory and concentration issues along with moodiness, paranoia, and hallucinations. It may also however lead to increased energy and alertness (paradoxically enough) and enhanced mood (being used as a treatment for depression).