Actually, I was surprised you were surprised :) I always wear my watch while sleeping, never seen a problem in that. But then I realized in other comments that

Actually, I was surprised you were surprised :) I always wear my watch while sleeping, never seen a problem in that. But then I realized in other comments that you were not alone in that. My bet: people who remove their watch for sleeping are the ones who tend to wear it tight, while people keeping it wear it loose.