It's general wear and tear over the course of the day. Caffeine, sodium, stress, etc. all make it worse.
You really have to be super conscientious about diet, exercise, and stress to help keep it in check until the find the cause of it or it resolves itself - and yes it can resolve itself. I'm great from when I wake up until about 3pm, at first it's quiet enough that if it's all it was I wouldn't even bother seeing doctors. 3pm is picks up a bit, but still not bad. 8 pm on is when it's usually worst. Just in time for going to bed, yay me. So I take 200mg chelated magnesium and 1mcg time release melatonin to help sleep and it works.
Sitting and laying down for me makes it worse because I likely have MCS - microvascular compression syndrome. Same with being in quiet places. A little noise as a distraction is actually good for you so just leave the stereo or TV on low even when you sleep.
And the better you do on diet, exercise, stress management the less intrusive your PT will be.