I track my sleep manually because at night the watch hurts my wrist. A few months ago I started noticing that the app and website sometimes wouldn't let me enter my sleep time.
I've figured out that it's when I'm trying to add times for a previous day. Sometimes I forget or I'm in a place with no Internet connection, so I need to add sleep for previous days. I used to be able to do so.
Around the same time I noticed a new badge for tracking sleep 30 days in a row, so I assume the change is so that people don't "cheat." But that means you've removed functionality for the sake of fun perks, which sucks. I know most people probably track sleep with their watch, not manually, but now I have to figure out how to track my sleep in a different app or on paper if I forget or can't sync, which is ridiculous when it worked fine before.
Please consider giving back the functionality to enter sleep manually regardless of what day it is. Surely "fixing" the possibility that someone will get one extra point every 30 days by cheating isn't worth making the app less functional.