Some characters stay up very late at night. I've seen some that don't wake up until 10 AM and stay up until 11 PM.
Check the time on your Tamagotchi, to see if it's set correctly. Also make sure the AM/PM is set correctly.
If the time is set correctly, and s/he seems to want to stay up past your own bedtime, set the time to 1 hour ahead, and see what happens. For example, if it's really 9 PM, set your Tamagotchi's clock to 10 PM. Usually one hour is enough, but if that doesn't work, try two hours difference. 9 PM --> 11 PM