There are fundamentally three different kinds of sleepers amongst humans.
The morning larks - These are people who like to go to bed early and wake up early as well. They like to sleep (9 -5)
The Night Owls - The late nighters who like to sit up the whole night and go to bed late and wake up late as well. They like to sleep (2-10)
The Late evening People - They don't go to bed as early as the morning larks but sleep earlier than the night owls.
The reason for this is apparently evolutionary. The difference in the sleep timings of the two means that There is only about 3 hours between 2 AM and 5 AM that a human settlement would have been completely vulnerable to an attack. Big cats like to hunt just after dusk of just before sunrise, hence relatively very few predators would be out hunting between 2 and 5 in the morning.
Modern society is very unforgiving of Night Owls because western societies established the factory life which starts at 9AM. Is this right?