Chocolate isn’t just higher in sugar and fat content than other late-night snacks. It also contains caffeine. If you’re sensitive to this stimulant, chocolate could rob you of the precious sleep you need to allow your body to recover maximally from your hard workout sessions.
As a result, you won’t make the progress you otherwise could. Those who don’t sleep enough on a regular basis are much likelier to suffer from increased cortisol levels, which will actually encourage muscle tissue breakdown.