Why do we sleep during night not during day

• Iam not saying about sleep during day during our class lecture and a superb meal,iam saying about the night sleep.
• It is due to the 24 hours biological clock that runs in the centre part of the brain know as circadian rhythm.
• When the world goes dark our light sensitivity cells of the retina feeds signal into the hypothalamus a part of brain .
• Hypothalamus in turn release a hormone called MELATONIN to the bloodstream from the pineal gland.
• It makes our body tired,sleepy,it reduces our body temperature and heart rate ...to make us sleep.