Nightstand Mode is a cool new feature in watchOS 2 that puts a nice fat digital clock on your Apple Watch when it's being charged. It's tasty, but some folks might not want their Apple Watch's display on all night. Turning it off is simple, but only if you know where to look. Spoiler: Settings > General > Nightstand Mode.
That's the short version, but here's what it looks like with handy-dandy screenshots. First, press the Digital Crown on your Apple Watch once to pull up the app screen. Then tap the Setting apps shown below.
The Settings App in watchOS 2
Next, you want to scroll down to the General section and tap it, as shown below.
General Settings in Settings App
Now, scroll down to Nightstand Mode and tap, as shown below.
Nightstand Mode Settings in Settings App
By default Nightstand Mode is turned on, as shown in the screenshot below.
Nightstand Mode On in Settings App
Just tap that setting to the off position, as shown below, and your Apple Watch will go back to featuring a blank screen when it's charging.
Nightstand Mode Off in Settings App
That's it. It's stupid simple, but I wasn't sure where to turn it off until I asked the brain trust that is TMO's staff.