I've heard that human's body time cycle is 25 or 26 hours, it's not 24 hours. That's why it's easy to stay up late.
Yuki : I am not sleepy today.
Nyankichi sensei : let's sleep soon...
Frog : zzzzz
Thinking about how the human body influenced by earth's time cycle, for example summer time, Feburary 29th, the movement of moon etc etc is amazing.