Just some pet theories and observations of mine.
Pet theory 1: it's possible to end up in a state where the sympathetic nervous system is constantly in some state of activation. Doesn't have to be full-blown but can be a constant sense of tenseness, etc.
Pet theory 2: parasympathetic nervous system (rest and digest) activation can be trained. As one gets better at engaging it regularly it will be easier to get into a state of parasympathetic nervous system activation.
Do I currently have the energy to put in the proper research? No. So it'll be "supposedly" and this is based on personal anecdotes.
Move eyes to one side for 30 seconds or until feel relaxation begin. Hold. Repeat on other side.
Not sure if this was directly stated to affect the parasympathetic nervous system, but a stress expert recommends it. Place your focus somewhere in your body, like the end of a limb. Slowly move the focus (up or down). Really focus and try to feel the sensations on that spot. Could be a full scan of the whole body or just a limb or two. Or a limb repeatedly. Either seems to work for me once I get sufficiently focused. I didn't believe this would do anything but it can be extremely relaxing?
A neurologist explains this in a video but I don't have the energy to look for the video. As far as I can recall the default mode network is the part of the brain that reflects upon stuff and puts things into context/narration.
Meditation seems to weaken the effect of the default mode network (I think there were studies done on this? Again, lacking the energy to check). At any rate, it's the most sensible explanation I've seen for why meditation works to date. Apparently it also is a sort of training your focus? But I'm definitely happier when my default mode network isn't babbling constantly, and meditation seems to have reduced the amount of rumination my brain does.
Thought catching (source: a youtuber) was also really helpful. Every time a thought that got me down happened and I'd examine and question and offer up a healthier interpretation. Don't remember the exact steps. Note: look for the video sometime, it's probably useful to have.