Are we suppose to wear our watch at night? 24/7?

You have to charge it. The Apple Watch is only rated for 18 hours average per day before it needs recharging. If you use it lightly, few phone calls, no music streaming, etc... it may last longer, but it must be charged on a regular basis.

The best time would be while you are taking your shower and performing other morning and/or evening bathroom activities.

An Apple Watch Series 7 with a 20watt Fast Charger may be able to be charged in 30 to 45 minutes.

A Series 6 or older is going to need about an hour and a half of charging time. Whether you do that all at once, or twice, or more times a day to spread it out to times when it suites your life style, is up to you.

That's something better asked of the researchers.