What is a mouse thinking?
Awake at three, running the wheel. Asleep whenever you are watching.
Mice are nocturnal and the wheel is not just exercise. Given one, a mouse will run several kilometres a night - wild mice will even use a wheel left outdoors, which suggests running is something they want to do rather than a captivity artefact.
The daytime sleeping is the same clock working normally. A mouse takes many short sleep bouts rather than one long one, which is why it always seems to be asleep exactly when you look in.
Try it on a real one
Three peeks a day, free. No account, nothing to install.