Upper Airway Resistance Syndrome is extremely common, and extremely underdisgnosed both - most sleep studies only screen for sleep apnea, and most physicians aren't aware of the different ways in which UARS presents, even though it's fundamentally the same disorder, and often as or more severe.
Basically, if you have any difficulties breathing through your nose, recessed or very narrow jaws, tend to breathe through your mouth at night, and your sleep is disturbed, you're likely to have either OSA or UARS, the former more common in men, obese people, and the elderly, while UARS predominates in women and younger, thinner people.
The difference is basically in how your body reacts to the airway collapse- whether you keep sleeping until oxygen desaturation happens, or wakes up to re-establish airway patency. Long-term, the hypoxia does more damage, but in terms of immediate symptoms, the worse fragmentation is worse.