Fitness tracker to monitor stress and being run down

i have never posted anything on any forum before so I hope I am doing it right. 

I have been using a wrist fitness tracker that monitors my heart rate, heart rate variability and sleep. The bit that interests me most is the “body battery” and “stress” measures. I was pretty sceptical to start with but I have found that I cope much better with stressful situations when my “body battery” is high. When it is low I am best to have some quiet time.  It has also been useful to look back and see when I have been most stressed. 

I am on the autistic spectrum myself and work with adults with intellectual disability and autism. I am thinking that this has potential to be be really useful. 

Has anyone else found a fitness tracker useful for monitoring how stressed or how run down they are?