I'm not sure of specific changes. I know that I'm more likely to have insomnia after driving to town or socializing. It feels like those activities rev my brain up, and it doesn't ease off quickly. Mostly I just try to be aware of patterns: if I get insomnia, I check my food/activity/symptoms diary and see if anything the previous day might be responsible. If I keep having insomnia following something specific, I avoid it. One problem is, it might be something so common or unlikely that you don't bother noting it, or it's something early in the day so you don't connect it. Playing 3 hrs of intense computer games before bed would be an obvious cause, but maybe doing your online reading at a certain time of day triggers 5AM insomnia. Maybe it's the artificial flavouring in your toothpaste.
Some of the difficult things to test or even consider are things so common that you don't think of them. I wondered about a foam mattress, possibly outgassing (nope, wasn't that). Carpets, furniture, building materials: I wouln't be surprised if someone in the world gets insomnia because of chemical sensitivity to something like that. Maybe something in your home generates a sound or smell that triggers brain activity. Of course, maybe it's nothing external, but an internal problem instead. Checking foods, activities, and household stuff are just something we can do ourselves.
One simple test is to try sleeping somewhere else for a few nights, to rule out the household stuff and normal daily activities. That might be convenient for some people, and very inconvenient for others; I'm just offering that as a possibility. If the lack of good sleep is a major problem, and you've ruled out the simple possibilities, there are sleep clinics. However, I think it's likely that the sleep experts won't take ME/CFS properly into account.
For myself, I try to find things that might be harming my sleep quality, and avoiding them. When I get insomnia before 4AM and just know I won't fall asleep if I stay in bed, I get up, have coffee (decaf) and read for an hour or so, then go back to bed and maybe will fall asleep again. After 4AM, it's not worth trying. If I absolutely need more sleep, I'll take melatonin or 5-HTP, but if I take it after 4AM it really messes up the rest of the day. Benadryl makes me groggy and confused, but not sleepy.
Hopefully you'll be lucky and realize that you just need to stop playing three hours of intense zombie shooting games before bed.