I doubt that a doctor would give you a script for less than 40 hours a week (a normal work week) unless it is medically necessary that you work less. I have found myself wishing for less hours at work due to stress, and considering this same option. However, I have found that the right medication lessens the stress, so I can work a full week. Work focus our minds on necessary things while making us the money we need to live. Another option would be to find a job more suited to your own personal work preferences. Hope this is helpful! :)
I think you might be able to get a psychiatrist to write a doctor's note to that effect, if he/she feels your mental health is in jeopardy working too many hours & hopefully, if your place of employment values your contribution to their company, they will take that into consideration.
If you are feeling too stressed at your job, please look for a less demanding job right away... I was stressed and a co-worker could tell, thinking I needed the stress management classes offered by the company. I wound up burning out & haven't been able to work for over 10 years now.
You can probably get a psychiatrist to write such a prescription, but it will be a faustian bargain. You will end up with a psychiatric history and will be number one on the list of "employees we don't want working for us". And don't expect a good evaluation of recommendation when you move on.
Overtime, unfortunately, in the United States, is necessary in many industries.
The best course of action would be to seek another job.
Looking at your post again, I see you mention medical reasons- are you suffering from a medical condition that is worsened from your overtime? If so, you might even want to consider filing for disability, which would involve a doctor prescribing time off- you can still work part time (if the doctor says it's okay) while you are on state disability- at least it was that way for me in California after a car accident.