Hypothesis:
Sleep is a finite resource that we draw from to fall unconscious.When there isn't enough sleep locally, you become insomniac.
That's why everyone can't sleep at the same time. Because of sleep shortage and not because some rock is tilted and light shining on some parts. You can sleep during the day, even in pretty bright rooms. The reason we get sleepy during the night is because of overabundance of sleep globally as the other side world wakes up.
Death creates new sleep consumed by people.