Routine rigidity, as it is most commonly referred to, is something that everyone experiences to some degree. You may drive the same way to work everyday, or you may like your coffee made just so each morning.
It's comforting, for a lot of people, to not have to think about the way they go about whatever routines they have established - brushing teeth, getting dressed, commuting, etc. When you spend all day thinking about work and family and life, to be able to rely on your routine sometimes so that you can turn off your brain for a while is a blessing.
But most people won't get "bent out of shape" much if their routine doesn't pan out as usual. A lot of people even go out of their way to mix up their routines sometimes, add in some spontaneity, do things a little differently than usual, for interest or inspiration. So why, then, do many people with ASD rely so heavily on their routines? Why does one thing out of place have the potential to throw my whole day into chaos? Why can I get so upset and inflexible sometimes when something doesn't go the way it always used to? Why do I experience "routine rigidity?"
For me, and for a lot of people with ASD, I think, routines are more than just comfortable. They are necessary. Because my brain does not autonomously do a lot of the things that other peoples' brains do automatically, I have to establish routines to help me bridge that gap. My routines don't just offer me a periodic break from thinking, they help me function at a level considered “normal“ by the majority of society.
Let me illustrate. A neurotypical brain - over the course of a person's entire life - uses context and intuition to create automatic processes which operate without conscious thought, "in the background" so to speak. This allows the conscious mind some room to breathe and relax. An ASD brain does not do this as much - I have to manually create my own "automatic" processes, which in turn, become my routines. For example, a neurotypical person will automatically smile at someone who is smiling at them. Their brain does it for them. They don't even think about it. Whereas a person with ASD may not have the luxury of an auto-brain process for this - if I smile back, it’s because I have manually built a sub-routine which I consciously access once I realize that someone is smiling at me. It‘s like entering an “if-then” command into a computer that has been programmed to respond in a certain way: “If see smile, then smile back.“
But then what if someone is smiling in an odd way, like a side-smile, or a wide laugh? Those require different sub-routines: "If smile big with laugh, A: then offer tentative smile while trying to figure out what is funny, B: then if what the other person deemed funny is not offensive or inappropriate, then laugh."
I must do all of this manually. Very rarely does my brain turn off. In order to finally relax a bit, I must have a routine established, and be ingrained in that habit many times over, before my brain is able to turn off and let the routine run on auto-pilot. As you can see, routines for me, for the ASD brain, are much more elaborate than just the particular way in which I get ready for the day each morning - they include sub-routines upon sun-routines which help me get through my entire day.
If you are neurotypical, it is probably hard to imagine building these micro-routines for everyday interactions, and then actively pulling them up when you need to use them. As an approximation, imagine for a moment that your brain does not automatically breathe for you. Breathing is an autonomic process that human brains do on auto-pilot, in order to help us survive. But imagine if your brain didn’t breathe for you - if you had to tell yourself to take each breath - if your breathing was not unconscious, but always conscious. Would your brain have much room left for anything else?
My conscious routines are not so dire as breathing - thankfully that is one automatic process my brain still does for me - but you get the idea.
The more I can do to help my brain work on auto-pilot in the background, the more I can use my mental power to focus on more important things, like being emotionally present for my family, solving a work problem, etc. If one morning my hairbrush is in a different place than usual, or if my son wakes up half an hour earlier than usual, my morning routine can go awry very quickly, and I am forced right back into conscious-thought mode in order to start my day. I easily forget things when I am outside of my routine, because everything is manual, nothing is automatic. So, move my hairbrush, and I might forget my keys when I walk out the door.
My routines are my way of making my brain do what a neurotypical brain does on its own - create automatic background processes that can run on auto-pilot while I spend my mental energy elsewhere.
Mess with my routine, and you may have no idea how hard I am suddenly working in order to keep up.