I just want to clarify that there is no such thing as a "protected disability" in the US. Disability is defined very broadly that it is something that inhibits your ability in a significant way to work and live. A sleep disorder absolutely can be a disability under the law in the US. But your doctor would have to state that it is severe enough to be considered disabling by the legal definition.

Any condition or disorder can rise to the status of disability and the ADA does not define any specific disorders that are included or excluded. So NO, they can't get rid of you bc of a sleep disorder, but unless you've disclosed the disorder and asked for accommodations officially at work, then technically they don't know and they can fire you for bad behavior if it's affecting your behavior.