Another way is to set up a sleep FOCUS in your iPhone for your normal sleep time (or longer).
iPhone settings > Focus > Sleep
and specify typical start and end times.
you can also enable text messages from certain users to push through sleep focus and be delivered anyway, if you like.
Then in Watch settings,
Watch > General > Focus > Mirror my iPhone = ON
[in my opinion] the most important change this gives you is that your watch stops lighting up every time you roll over -- (so often seen as 'raising your wrist'). If you actually do want to see the time, just tap the display .
if you do not choose a different face for this sleep focus, it reverts to the default black background with white digital time, rather dim.