how does an apple watch monitor sleep if you need to charge it

Unless you have an Apple Watch Ultra, your Apple Watch has to be at least charged up to 30%, to be able to track your sleep:

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If your Apple Watch is charged less than 30 percent before you go to bed, you’re prompted to charge it. In the morning, just glance at the greeting to see how much charge remains.

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copied from: Track your sleep with Apple Watch - Apple Support

The Apple Watch Series 7 and newer with Sleep monitoring also have Fast Charging

About fast charge on Apple Watch - Apple Support

0-80% in 45 minutes.

So if you have a 20watt charging brick, and a Series 7 or newer, then you can top up your Apple Watch while you are brushing your teeth, and getting ready for bed.

And in the morning, again put it on the charger while you are getting ready for your day.