With watchOS 2, there are two good options for what to do with an Apple Watch at night.
The first is to use the Apple Watch a bed-side alarm clock. watchOS 2 includes "Nightstand mode," which turns the watch into a tiny, adorable alarm clock when turned on its side and set on the charging puck.
As you can see from the photo, the Watch displays the time, day and date, as well as charging status. All of this is done in a nice green that's not hard on the eyes in the middle of the night. If an alarm is set, it shows as well.
The clever thing here is what Apple's doing with the display. The display stays on for several seconds after the Watch detects that it can enter Nightstand mode, then goes to sleep.
Thankfully, you don't have to interact with the Watch to see the time. Thanks to the built-in accelerometer, Apple Watch will wake up and display the time for several seconds with just a bump of the table its sitting on.
If you have an alarm set, the Watch uses a tone that's actually nicer than anything built-in on the iPhone. Press the side button to turn it off or the Digital Crown to snooze.
I'm using ElevationLab's Night Stand to keep my watch horizontal at night. It sticks to the surface it sits on, so it's not going anywhere, which is nice, but I've found that I need to strap my sport band together to keep the Watch from wobbling on the stand.
As nice as Nightstand mode is, David Smith has made the Watch a lot more useful at night with a new app called Sleep++ which can track how well you sleep at night.
The TL/DR is to charge your Apple Watch in the morning while you get ready for your day (take a shower, get dressed, etc) and then again in the evening while you get ready for bed (brush teeth, put on pajamas, etc). Then put your Apple Watch in Airplane Mode while you sleep.
I've been using Sleep++ for several nights now, and I have to say, I really like it.
Launch the app and tell it you're going to sleep. After that, at David's suggestion, I've been putting my Watch in Airplane mode. Here, you can see that I set it to go to bed at 10:09 last night, because I'm an old person.
The Watch's screen will still light up when moved, even at night, and I found that to be a problem, so right before bed, I've been knocking the screen brightness all the way down and selecting this variant of the Modular face I've setup to show just the time in red. Now, if I do turn over, the Watch's display shouldn't be bright enough to be an annoyance.
In the morning — waking up with a silent Watch alarm is nice — tell Sleep++ you're awake, and the app will analyze the night's information and put it on a chart shared between the Watch and iPhone app.
While I don't love the feeling of sleeping with my Apple Watch on, I'm already getting used to, and Sleep++ makes it worth it. How I sleep is an important part of my health, and one that I haven't been tracking since putting my Fitbit in the drawer. This app remedies that, and thanks to Nightstand mode, at least I have another use for the Watch at night if I want to skip a night or have goofed up the charging regime.