I do not think it to be exercise intensity, for the most part, because I, for example, have exercised intensely for many years,(about 30) 5 years with Cathe workouts 2 hours a day. My doctor, fully knowing my routine added more workouts a day...Pilates, Yoga, type workouts at the end of the day. But of course, of course, everyone is different. Unless you were jumping and running, to the highest of intestity and then immediately go to bed, I could see that the body would be hot-wired and need time to get into relax mode.
The "can't sleep" syndrome is something only the doctor can diagnose of course, but it usually is some chemical imbalance or sorts. The body is so very complex with its own electrical system, computer, chemical laboratory, etc. For me, one chocolate cobbler (fudge brownie, icecream with hotfudge on top) will send my electrical and chemical system in a huge whirlwind of hot flashes, turning red faced and a great big migraine. Just one. Not fair! LOL
The neural system, the part that affects the sleep mode in us, can easily go haywire with the wrong chemicals, which we can take in by our diet, but also the chemical changes we have just by getting older. There are tons of things going on inside us. I am in awe of the whole process of how we are made and how we function.
(sorry, I get carried away. I'm taking a short break from doing spreadsheets and cash flow statements for a farm corporation. Cannot wait to get done so as to get back to my MT schooling. Guess you can tell I love it.)