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.
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.
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.
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! :)