I agree. In the past, I’ve tried to just be disciplined, stop procrastinating, and push through no matter what, and it lead to horrible burnout where I spent weeks/months being fairly ineffective. If you’re not sleeping at all at night and can’t think straight during the day, even for a couple minutes, you’re simply not going to be effective. You’re also liable to make really bad decisions that will drag on you even after you’ve recovered.

Having a routine though, even a very flexible routine, has been very effective. Things become automatic. Settling into an effective routing often involves not worrying too much about discipline, though. It's easier for people to get to the gym everyday if the goal is "get to the gym, and do something" than it is if the goal is "get to the gym and do a rigorous 2 hour workout." Small steps and environmental changes, as well as not stressing over what's optimal, have been the most effective approaches in my experience.