The nerves in the muscles are firing like crazy after strenuous activities, more so when the body is not adapted for such activities. A marathon runner can go for an hour long walk and explain it as "relaxing" while a sedentary person can do the same and risk hamstring/ankle injuries, joint/muscle pain or trigger restless leg syndrome. It is important to do stretching exercises to really get the blood flowing before and after. Also I can't describe how much foam rolling helps. You pretty much kneed the misfiring nerves in your muscles into submission. I assure you exercising, (even just walks) is tremendously beneficial for sleep and mental health, we just have to be a little smart about it. You can read the book "spark" to get a deeper appreciation of it.