Because I spent so much of my energy processing sensory stuff that most people can just block out and ignore, I sometimes run out. It's not that I have less energy than a neurotypical person, but I blow through it more quickly.
In general this is something I just work around. I know I can't go out more than one night through the week, for instance, or I'll be exhausted for days afterwards.
But at the moment I'm crashing, quite badly. I'm sleep deprived, haven't exercised much lately, my diet's not great, and I'm well behind on the housework and paperwork and other stuff that makes up everyday life.
It all started with a couple of late nights watching QI (an old favourite obsession which has recently returned with a vengeance) on Youtube, and from there it's just snowballed into a general state of apathy and not getting stuff done. I'm tired in the afternoon so I have a nap instead of going for a walk, then wake up too late to cook a proper tea so I don't eat properly, and am alert past my usual bedtime so I go to bed late and don't get enough sleep... which means the next day I'm tired again, so the whole cycle repeats.
In one way it's a snowball, but in another it's a house of cards. Dislodge one card (a few late nights, in this case) and the lot comes down and you have to start building all over again.