I've found that people who stay up late also tend to tell you [edit: and brag] about it. For example, if a friend is out late partying you're almost guaranteed to hear about how late they stayed out the next day.

I think it goes both ways because people want to justify why they are not at 100% energy. Partly to not offend others when you're being abnormally unenthusiastic. A type of social defense mechanism.