Our body follows schedule/pattern within 24hours that whenever the body is not able to comply to it, something happens.  This pattern/schedule is called circadian rhythm, which is defined by wikipedia as  “a roughly 24-hour cycle in the biochemical, physiological, or behavioural processes of living entities”.

As circadian rhythm indicates, the best time to sleep is from 10PM, because it is  when the body tissues repair.  Did you notice that when you sleep late, even though you have completed 8 hours, you don’t feel rested? To those whose work schedule is at night, even though they sleep more than 8 hours everyday, their health is affected, to some extent, anemia may take place.

See circadian rhythm diagram below to understand how the body clock works. I believe circadian rhythm follows the time where a person lives.

taken from Wikipedia