Can I be fired at work for an undiagnosed medical condition?

I fall asleep randomly at all times of the day. Are there any laws that protect me against being fired? The onset is very sudden and I have no chance to get up and walk around or anything else to keep myself awake. I have been under a doctors care for over two years and they can't figure out what is wrong. We have tried diet, different sleep times, a sleep study, and everything else you can imagine.