It's not in the regulations. It's only in the Guidelines for Physicians. If the particular examiner feels you need a stress test, or a sleep test, in order to sign off on you being physically able to perform the job, he can require it. The Medical Advisory Board that comes up with these guidelines is of course comprised of doctors who specialize or are financially vested in things like sleep apnea clinics and stress test facilities.
Yep....If you meet any or all or any combination of medical issues....A Dr, can be held responsible for letting someone go untested....some of my Dr. buddies are NOT offering certifications because of the liability issues...
http://nrcme.fmcsa.dot.gov/mehandbook/about_391.43_ep.aspx
FMCSA Medical Examiner Handbook
Part III - Medical Examination Guidelines