From the American Sleep Apnea Association:

There are three types of apnea: obstructive, central, and mixed; of the three, obstructive is the most common. Despite the difference in the root cause of each type, in all three, people with untreated sleep apnea stop breathing repeatedly during their sleep, sometimes hundreds of times during the night and often for a minute or longer.

Obstructive sleep apnea (OSA) is caused by a blockage of the airway, usually when the soft tissue in the rear of the throat collapses and closes during sleep. In central sleep apnea, the airway is not blocked but the brain fails to signal the muscles to breathe. Mixed apnea, as the name implies, is a combination of the two. With each apnea event, the brain briefly arouses people with sleep apnea in order for them to resume breathing, but consequently sleep is extremely fragmented and of poor quality.

Untreated, sleep apnea can cause high blood pressure and other cardiovascular disease, memory problems, weight gain, impotency, and headaches. Moreover, untreated sleep apnea may be responsible for job impairment and motor vehicle crashes. Fortunately, sleep apnea can be diagnosed and treated. Several treatment options exist, and research into additional options continues

The ASAA goes on to say that sleep apnea is very common (as common as diabetes) yet many go undiagnosed. As Ron said, his wife told him that he gasps for air; the same was true of me and my husband. Hubby told me I wake him up gasping for air, none of whicih I remember. After he continued to complain, I had a sleep study. They hook you up to all sorts of wires (heart, brain, blood pressure, etc. and you spend the night in either a hospital or sleep clinic. You aroe monitoreod as to how many times you gasp for air and actually stopping breathing for short periods, among other readings taken.. I had my study 5 years ago and stopped breathing 40 times in a 5-hour sleeping period. That's not real bad, but bad enough to require a CPAP machine (continuous positive airway pressure).
Peg
The downsid (in my uneducated opinion) would have to do with the brain receiving proeper oxygen levels