Agreed on getting a medical diagnosis before going to HR. I get serious migraines that can be triggered by fluorescent lights, and I have a letter from my neurologist saying I need a work environment without fluorescent lighting. HR made it happen, but I don't think they could have justified the expense without the letter.

I don't know whether ADA or another law covers migraines. I'm lucky that it didn't come up: my company has a policy of providing medically necessary accommodations even when there isn't a legal obligation. Requiring a doctor's letter helps them keep such a policy because it avoids putting HR in the awkward position of adjudicating what constitutes a medical necessity. HR considers something to be a medical necessity when a certified medical professional says it is.