I started studying regularly to exercise my brain and since then I have noticed a shift in my symptoms. I've started to notice all the noise coming from outside and how uncomfortable it is. Dogs barking, distant and near traffic, people talking, music in the distance, machinery humming, wind, flowing water. All of it often feels like unpleasant stimulation. My sleep has gotten worse, maybe due to this sensitivity to noises which continue throughout much of the night. With poor sleep, my ability to function plummets.
I also seem to have a harder time relaxing fully, which makes sleep less restorative.
Ugh, sorry this has happened.
I hear you. What I've found for me is that ideally I only do cognitive things in the morning. If I let it leak into the afternoon, or even worse, evening, I really pay for it. I'm being bold at the moment, and I am paying for it.
I find that doing anything cognitive sets your mind working on it for the rest of the day. So your brain is continuing to exert more than usual even if you stopped hours ago. This is really apparent during "rests", if I can still call them that, during a time when I'm doing more cognitive work than usual. The whole rest is my brain working away on whatever I was doing that morning. Really not restful. And the common phenomenon of waking up having worked something out.
I also need rest days.
If I'm strict with mornings only, things are much better, but I still need rest days. If I'm doing more cognitive work for a while, I will eventually get worse and worse brain fog and need a long break. So if I were at your stage and aiming for doing something like study consistently, I would need to make it every second morning for max X hours, with weekends off, or something like that.
Editing to add: Also worth considering posture while studying - if you're studying sitting upright in a chair, and usually you wouldn't sit upright in a chair for that amount of time, that could be contributing. If I poke my orthostatic intolerance, I get symptoms like those you've described too.
Hope you can find a way of doing a bit of study without messing up sleep and rest.