Psychiatrists make excellent sleep physicians and are really needed. I would argue OSA is the easiest of the major sleep disorders and is largely going to home testing. The techs diagnose it correctly 95% of the time.
Insomnia is way more common and a lot more difficult than bland OSA. Reading full montage studies for parasomnias also takes more skill, not to mention the subsequent treatment. Pediatric sleep and narcolepsy can also be challenging and interesting.
"Dreams permit each and every one of us to be quietly and safely insane every night of our lives." - William Dement