Help with Medical Restriction

Hey all, so I have a medical restriction but it's more a precautionary restriction.

Does it have to be applied 24/7? Before, management would ask me to break it, and I would. My Union president told me when I break it, I'M the one who's liable.

The restriction is just so I don't exceed 8 hours a day. I'm like hyper sensitive, very prone to migraines, most days I'm fine but some days it goes over my limit which is why the restriction was put in place by my Doctors. They're basically trying to make sure I don't have stressed induced illnesses.

If I agree to it, can I choose to work over 8 hours? I know management will be snippy the days I want to only do the 8, so should I just stick to that? I just feel like if I only do the 8, scheduling would be hell for everyone involved. I just don't know how to talk to my management about this...