Until you slip up - because e.g. you gave yourself too big a quantum of work to do before your next reward, or something unexpected popped up that you need to deal with ASAP. Then you'll find yourself tired, hungry, frustrated and in a downward spiral - because you'll be quite likely making mistakes sabotaging your work instead of making progress, thus putting off your eating time further in the future.

And I'm not talking out of my ass; I screwed myself a few times like that with the whole "precommitment" idea, including losing quite a bit of money - instead of helping me focus, this only stressed me out further as I was desperately thin on funds.

Not saying precommitments are a bad idea, just make sure you can handle failures mentally before going full-Klingon on them.