How do you decide what kind of work to schedule when you’re already mentally drained?

I’ve noticed I’m bad at matching my energy level to the work I plan.

I’ll block out time for deep work, but sometimes I only realise halfway through that I’m too mentally fried to do it properly.

I’m curious how other devs deal with this in practice:

Do you actively plan your day around how you feel or stress level?

When a day goes “off”, what usually breaks the plan itself, or your ability to follow it?

Have you ever tried time-blocking or similar planning methods? If so, what didn’t work for you?

I’m exploring a small personal tool in this space and trying to understand the real pain point. not selling anything.
Thank you all in advance for your reply to this.