I've known many people who do, but I couldn't ever understand it. It seems more common with people who have fully waterproof sport watches, I assume at least in part because they can wear them in the shower.
My LG G and my Pebble before it integrate with the alarm app Sleep for Android. Basically, I set a "I need to wake up by X time" in the app, the app uses the sensors in the watch to monitor how heavily you're sleeping, and will actually set off the alarm at the point within ~a half hour before the alarm time where you're sleeping lightest.
For someone like me that has a very, very hard time getting up in the morning, it's a fairly life changing thing. I also have Sleep tied in with a set of Philips Hues in my bedroom to have it start ramping up the lights to wake me up.
I'm still not a morning person, but I'm less likely to sleep straight through my alarm in the morning these days.
I just woke up after eight and a half hours of sleep, and had 88% battery left. Now, this was also with the Watch in Airplane Mode, but I've forgotten some nights, and still woke up with battery percentages in the same ballpark.
(Data point: I wear the 42mm model. The 38mm has a smaller battery.)
What's there to understand? It all comes down to whether the watch troubles you at all when sleeping. And if it doesn't bother you during the day, why would it bother you at night, when you are mostly unconscious?
I sometimes wear it (during sleep), and others not. Analog watches I can't usually wear when going to sleep, because I have my wrist close to my head, and the "tick tick" noise annoys me. Digital watches, no problem.
>It seems more common with people who have fully waterproof sport watches, I assume at least in part because they can wear them in the shower.
Huh? You can just wipe the watch every now and then.
I can't sleep with one on, and I don't understand why it doesn't bother others who can.
> You can just wipe the watch every now and then.
Even if I could sleep with a watch on, not having a sport watch means that I would have to take it off every morning anyways, so why not just take it off at night?
What's there to bother them? It's just something you have on you wrist all day anyway and barely notice it when you're not checking the time.
I guess some people can't sleep with socks on or pajamas either, but it's not something that's anything more than a personal curiosity.
It's not like having a watch on while sleeping brings in any pain/noise/other issues that make it objectively bad for sleep.
Your watch does not share those properties.
(And even so, lots of people, have no difficulty sleeping with their shoes or even boots on -- on the sofa at home, in an airport waiting, and it tons of other places besides, and even on the bed, usually with clothes on too, when dead tired).
Yours is a very weak argument, find a better one.
Even with my giant stainless snowboarding watch I'd be worried about breaking the watch while I sleep.