How to track short naps with Apple Watch?

You can’t. Because it doesn’t even actually track sleep. It only tracks when you’re laying in bed and if you have a morning alarm and you stop it, it assumes you have woken up and stops tracking sleep. It is terribly inaccurate and I’m going to switch back to Fitbit for sleep tracking.

See this;

Track your sleep with Apple Watch - Apple Support

Tap on the blue text above.