I think that has to do with the base stress level.

This morning, for example, I was extremely sensitive. I was at chemistry lab, and the smells nearly made me throw up. The sound of lab glassware being used felt like explosions in my head; I had a lot of trouble distinguishing speech from background... You know, the usual.

If I had overload that bad every day, I wouldn't be able to live independently, plain and simple, because I'd spend so much energy dealing with it that I wouldn't be able to care for myself.

Most days, it's not that bad. This morning, I was sleep-deprived, hungry, and worried about a test. That's the base stress level. On top of that, even mildly annoying sensations can really overwhelm you.

Lots of people waste time looking for direct meltdown/shutdown "triggers". They're only looking at half the picture. The overall stress level, too, is huge. If you're sick, if you're tired, if you're depressed or anxious or you haven't had enough time to relax (60-hour-a-week ABA, I'm looking at YOU!), you won't have the mental resources to filter all that incoming information, and sooner or later it will just turn into a tsunami and wash you away. Even worse if people have been trying to suppress your stims. Those are how I regulate sensory input, and I'd be a lot worse off without them.

I distinctly remember going into total shutdown one day last quarter because I was walking next to somebody whose shoes were squeaking. If you look at the surface, you think, "Oh, squeaky shoes, that's a meltdown trigger, we have to get rid of that or get her used to it." Well, there have been days when my own shoes squeaked and I had no trouble with it. I remember that day, I was tired, had a cold, and had had to sit through a two-hour commute on a loud, bouncy "assisted transport" van. If it weren't for that, there would have been no problem with the shoes.