You’re not slacking off if you put your well being first.
During preflight safety demonstrations, flight attendants tell you to put your oxygen mask on first, before trying to help anyone else. You’re not much use to someone else if you’re compromised too.
Most of do it or have done it, skipping meals until a project is done, staying up late then getting up early, saying you’ll do some exercise later.
It’s too bad that lots of people treat their well being as a sort of reward for something else they’re working on. People often don’t realize they’re short changing themselves, taking care of yourself shouldn’t be seen as a reward. It’s part of the process of being well which might help you better do those other things.