Depends on how overstimulated and what type of overstimulation ^.

For low level longer-term overstimulation - too bright light, for example - I often end up with a headache and worsened concentration. If I leave it for ages it ends up being more migraine-ish and I have to reduce the stimulation. This also gets worse when tired.

Often I can end up with low level overstimulation from being out on a bright day or if it’s too hot/cold/loud >.< - to deal with this I often physically shut my eyes for smaller periods until I find somewhere more shady or with less noise - even if brightness is not directly overstimulating, it’s more like there’s a “total amount of sensory processing per second” and too much stimulation overfills that bar for all senses, it’s like everything becomes brighter/louder/etc.

And I start getting a headache, and my response/reaction time gets slower because I’m spending more thinking cycles dealing with the sensory input >.<. Long term overstimulation can make me pretty irritable as well.

For more sudden overstimulation - certain types of noise in particular, usually white noise of sufficient loudness for me, or extremely bright light flashes - it hurts a lot, kinda feels like being stabbed in the brain repeatedly. If it goes on for more than a few seconds without any way for me to moderate it I just stop being able to respond to any complex input (other person talking) because I spend all my effort trying to cope with sensory input >.<, and I will just do anything to get away from it. I end up mute/non-verbal from overload (speaking is way too much processing), and have trouble responding to complex external stimuli without extensive delay or not at all.

I am still thinking, but very “stuttery”/incoherent, and all my (small amount of) remaining thinking capacity is going towards any way to leave the physical location or halt the stimulus. I can lose my mind’s eye in this situation (or rather my ability to concentrate on it), as well as partially lose my internal narrative. It’s also just very panicky and constant, excruciating pain, though I often dissociate a bit from the pain.

(On that note, stuff like this is one of the several reasons I hate ABA. Pressuring people (“positively” or negatively) to endure that kind of pain is disgusting and cruel, and the organisations posting videos of autistic people experiencing that stuff to promote it is even more horrible to even watch, especially since the thing they promote directly contributes to that pain by pushing people to suppress other autistic mechanisms for emotional and sensory self regulation, like stimming ^. - hence I don’t willingly watch those things)

This in particular is bad for me with fire alarms. Fuck fire alarms with every fibre of my being. Covering my ears barely helps, they are excruciating beyond what I can describe. For things like food with bad texture (mostly olives), or bad textures in general, it’s easy to remove myself, but if I was coerced to endure it the effect would be similar >.<

Often I also dissociate when overstimulated, and this is in response to continuous hostility when I was younger for my more outward autistic behaviours so I just dissociated 24/7 to avoid expressing anything until I reached a breaking point (I won’t call it masking since I never actually successfully hid my autistic traits, just ended up with a weird traumatised version of them that I am still unpacking :/).

When I was younger this was more of an emotional overstimulation thing too - these days I have a decent handle of the emotional part now I let myself stim properly and am out of a very fucked up social environment, and am not cripplingly sleep-deprived every day, and can pick and choose to just remove myself from situations (social ones, anyway) that are causing me extensive stress.

Overstimulation is tiring so I often like to shut myself in my room and turn off all the lights, close my eyes, rest, etc. after it ^.