I work in the Sleep Medicine field.
For private and commercial vehicle holders with a diagnosis of OSA, a Sleep Physician report is needed to indicate that the patient is compliant with effective therapy for OSA. This either means CPAP or some other therapy. In this group, a report is required annually from a specialist (not a GP), e.g. Grafton RTA medical unit in NSW for example spits out a form every year to put people through the ringer, regardless of whether it is medically indicated or not. Some patients have had resolution of their OSA with weight loss confirmed with a sleep study, but still receive the RTA green form - once on - you're on for life.
A national document from Austroads is often referred to (on PDF online).
For PPL, pretty much the same process as above, but I suspect every 2 years for class 2 although not sure, could be annual.
There should still be concern about OSA though, but with balance - tiredness and fatigue are on a spectrum and affects people in different ways. For fatigue related accidents, a microsleep is not required, as the disturbed sleep from OSA can impair reaction times, tracking and judgment.
A vexed issue for commercial truck drivers - inadequate screening and no incentive to seek medical attention as automatically singled out and earmarked for life. Stats are bad - more than 50% compared with 10% general population, but most avoid diagnosis. Commercial pilots look a healthier bunch.
A broad sword approach to all-comers is just stupid. CASA and RTA should respect medical judgment - the example above of annual CT scans indefinitely for a kidney stone is negligent at best. Knowing you're going to be screwed over by a regulator that does not listen to specialist medical advice does not inspire confidence to seek medical advice in the first place- places safety in jeopardy for the long-term. Motivated by self-preservation and easier to say "computer says no" I suspect.
Read in one of the American flying mags their regulator was considering mandatory sleep screening if weight/BMI was above a certain limit. They're not happy.
First victim of bureaucracy is common sense. Medicos now have to routinely negotiate with dim-witted genY public servants on a daily basis to manage patients. That felt good typing that sentence.