I don't really understand the sleep? Is this for people with apnea trying to monitor without a sleep study? What's the draw for a sleep monitor? Seems like a feature with even less demand than "breathing".
I think it's because most fitness bands have that sleep-tracking option. I've never seen the point of it. I usually can't force myself to sleep longer unless I exercise like crazy in the daytime. No product appeals to everyone, but I'm sure it's a function of software to measure sleep by way of less movement, so Apple could likely offer that feature if they thought it was that important.