Actually I am a supervisor with the Postal Service and am well versed in actions such as this. There is a thing called FMLA (Family Medical Leave Act) which covers employees for extended periods of time being unable to work due to a serious health issue or one which will not get better over time. FMLA is the type of leave which is utilized by women when they are out of work when they have a baby, it is not widespread knowledge that this same type of leave can be used to protect someone, even a male, with a serious health issue. You would have to have your doctor document the serious condition and then your human resources or personel department should be able to run with the ball after that.
The Family Medical Leave Act once again is not just for pregnant women and it is the law, not just a postal service policy but a law covering an employee for up to 12 weeks of absence at work. Mind you this does not mean you receive PAID leave, depending on your predicament, situation, and career you may be able to use your sick, vacation time, etc BUT FMLA only bereaves you from any disciplinary measures which your employer may have taken against you for your time not at work.
In addition to sleep apnea I have crohn's disease and this is an example of an FMLA covered condition which I can not be disciplined for when I am unable to attend work.
If I was FIRED or any action taken against me for my health conditions I would immediately consult a labor lawyer. In addition to getting your job back you will also walk away with your self respect.
Hope this helped