We don't sleep when we should

Many people live like shift workers to some degree. The majority of people either go to bed after midnight or wake up early without getting enough sleep.

Shift work is a probable carcinogen, according to the WHO. Moreover, there is a list of health problems associated with shift work. The reason is that shift work interferes with your circadian rhythm. Humans are diurnal - we are designed to be awake in the day and asleep at night. While we can be nocturnal, we weren't built for it long term.