Yes.
You can manually activate Sleep on your Apple Watch or on iPhone in Health app.
From Track your sleep with Apple Watch - Apple Support :
Set up Sleep on Apple Watch
- Open the Sleep app on your Apple Watch.
- Follow the onscreen instructions.
You can also open the Health app on iPhone, tap Browse, tap Sleep, then tap Get Started (under Set Up Sleep).
Note : Your Apple Watch will track your sleep based on the sleep schedule you create or when you manually turn on Sleep Focus. ToIn order to receive sleep data from the Apple Watch, it must track your sleep for at least 4h each night / each time you sleep.