I was diagnosed with obstructive sleep apnea about three years ago,
and have been on a CPAP machine ever since.  In plain language, when I
fall asleep, my throat tends to sag shut, with the result that I
temporarily stop breathing.  I then let out a gasp, waking up most of
the way in the process, then, as I go back to sleep, the whole process
repeats itself.  The Continuous Positive Airway Pressure machine
delivers forced air through a face-mask (I use a "nasal pillow" type,
meaning that it plugs into my nostrils), keeping the airway open so
that I can get a normal night's sleep.

Lately, I have started having problems with daytime sleepiness again,
suggesting that I am not getting full benefits from the machine.  I
have an appointment with the specialist on Tuesday, and he will decide
whether or not I need to go through another full sleep study.  Since
there have been a few occasions when I have awakened to find that the
mask has slipped to one side, it may be that I will just need some
adjustments to the mask itself.

Incidentally, if anyone in the group has untreated apnea, you really
need to have the treatment (generally a CPAP machine or surgery).  A
recent medical study found that patients with untreated apnea have a
five times greater chance of developing coronary artery disease,
strokes, and diabetes than do treated apnea patients.