Yes sorry I did not explain this definition of stress. I don't mean stress in terms of when you're worrying about something or you have too many jobs to do. Those are kinds of stress, but I mean in a much broader sense than this.
I have been reading/listening to Stuart Shanker recently and his definition of stress in medical terms is something that causes the brain to use glucose more rapidly than normal. We use glucose constantly, this is basically the "fuel" of all brain processes. So everything from maintaining essential functions like breathing and digestion, to sensory processing, to concentration, to socialisation, to physical exercise. All of these are stresses on the body.
Of course these are all normal processes so everyone is doing them, that's why at the end of the day you're tired and you need to rest, recharge, sleep, eat etc - and anybody would be tired after an extra-active day or one where they learned a lot, socialised a lot etc.
The theory is that when you have some kind of difference causing some of these processes to be harder (e.g. a motor delay making even walking much more tiring than it should be, a speech delay making forming speech quite a concentrated process) or to "take up" more mental energy or bandwidth (e.g. a hearing difficulty so not only are you processing sound you are processing it twice as hard, a sensory processing difference so you are less able to screen out unwanted stimuli such as background noise, an undiagnosed food allergy that is giving you tummy aches all the time) or if there is something environmental which is contributing (e.g. a child living in a chaotic household not getting enough food or sleep, a child worrying about mummy and daddy fighting, a very noisy classroom with a flickering lightbulb) this can both be taxing on the brain in general and in neurobiological terms, it literally uses more glucose meaning there is actually less resource available to spend on things like concentration, learning, impulse control, emotional regulation etc and then that is a self-perpetuating cycle, so children under a higher stress load are not learning as well, and then because they are not learning as well, everything becomes harder so it is even more stressful, or they are engaging in disruptive behaviour and getting told off, which is stressful, and over time you would then see a cumulative effect. This can be learning in the sense of school subjects, or we could be talking about learning at a very early age, the kind of learning toddlers do when they explore the environment, experiment with objects and begin to interact with others and observe the results.
A lot of people are talking about screen time and while I don't think you can make it as black and white as saying screens are terrible/screens are harmless, in terms of this environmental stress, what they tend to say is that a child being quiet is not necessarily the same thing as a child who is calm. So parents (and I fully admit to doing exactly this) will give children screen time because they sit quietly and it seems that they are resting, but internally they might not be resting because their brain is busy processing all of this sensory input from the screen, which can also be quite addictive. The screen time of today (streaming/on demand, dedicated children's channels, youtube, tiktok, games, social media) can also be more stimulating than the kinds of TV programmes which were available to children in past decades. The institute that Stuart Shanker runs is actually quite concerned about stress levels in children generally because they say a lot of children that they see literally do not know what it feels like to be calm. They are continually (neurologically) in a state of high stress/high alert - and these are "typical" children without diagnoses. So if they are seeing that in neurotypical children then it stands to reason that autistic children, who are already likely to be using more of their resources on things like sensory processing, because they often take in more sensory information than the rest of us, and social interaction because social nuance can be difficult for them meaning that a lot of their social interaction is unpredictable, just as two examples, without taking things like speech delay into account, the effect is likely to be more severe in a child who has these differences to start out with. They aren't starting from the same point as everyone else.