What happens if a person who usually falls asleep before 10pm and is up between 6-6:30 am stays up until after midnight because of the New Year and related firework hoopla?
Well, that person probably still wakes in the early hours by habit.
Probably that person falls back to sleep because of the short night.
Then that person wakes after 9am, groggy, out of sorts, and under-caffeinated
Welcome to the new year!