This is also true for me.

I look at my energy level like a bank account. People, all people, wake up with a certain amount of energy to get them through the day. I wake up with much less than a person without a Myasthenic disease. It also costs energy to function during the day and it costs me much more than a healthy person. Something as simple as showering costs me a lot MORE than a regular person. I have to make sure I don't drain my "energy bank" too quickly or don't save/allocate enough to be able to get myself into bed at night.

It's a balancing act and sometimes no matter how hard I try, I mess up and drain my "energy bank" before I should causing me to be "down" too early in the day.