Your body becomes paralyzed during REM sleep. It's what keeps you from running around your house (or worse, outside your house) when you're dreaming that somebody is chasing you.

If you suddenly awaken during REM sleep, you may have a 15-30 second period of paralysis.

I've had it happen to me once when a phone rang. I was terrified. I couldn't move my body for about 30 seconds. It was a weird feeling.

Sleep paralysis doesn't explain sleep apnea. Your pharyngeal muscles can become relaxed and cause obstructive sleep apnea during all stages of sleep.