I think it has to do with lifestyle Wyowoman.
People get use to different sleep patterns because of their different routines.
Some people are creative in the mornings, others like to work at night. Then their are those who can not identify a pattern in their routine.
I also believe our nature influences this. Some people are night owls others are roosters
I like to Catnap, others hate resting Mid day
I believe we can change these patterns, but most people avoid change and just claim it as 'the way they are'.

Banjo Bonnie