I completely understand how you feel. When I stop training, I don’t sleep well, I’m grumpy and can’t sleep. Those neurochemicals that you mention affect us both mentally and physically in such powerful ways.
During rest periods, I don’t think you need to be completely sedentary in order to rejuvenate. I like to find ways to occupy my mind and body with activities that I don’t normally do when I’m training hard. Ya know, things that you enjoy and will be satisfying to you, but not terribly strenuous. Especially outdoor activities because there is something special about the chemicals released during activities performed out in nature.
I like to do some kind of yoga, stretching, mobility or strength training because those things help me feel more balanced physically, take the dog (or friends) for walks and long hikes in places I don’t normally run and MTB, and also cook, bake and strangely enough, clean and organize and take care of things that I’ve neglected during the summer. I know most people don’t WANT to do house chores, but I know those are the things that often get neglected during the summer when we’d rather spend time outdoors. Getting that stuff done can be cathartic and can provide some mental peace and satisfaction and help us feel better about knowing that we’ll soon be back to training hard and neglecting that stuff once again.
In addition, the breathing exercises in our yoga program might be beneficial, or maybe learn to meditate to help calm the mind and re-focus your energy?!
Best of luck!