Kyle

       Here is an abstract I found on PubMed.

Situational factors affecting sleep paralysis and associated hallucinations:
position and timing effects.

Cheyne JA.

Department of Psychology, University of Waterloo, Waterloo, Ontario, Canada.
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Sleep paralysis (SP) entails a period of paralysis upon waking or falling
asleep and is often accompanied by terrifying hallucinations. Two situational
conditions for sleep paralysis, body position (supine, prone, and left or right
lateral decubitus) and timing (beginning, middle, or end of sleep), were
investigated in two studies involving 6730 subjects, including 4699 SP experients. A
greater number of individuals reported SP in the supine position than all
other positions combined. The supine position was also 3-4 times more common
during SP than when normally falling asleep. The supine position during SP was
reported to be more prevalent at the middle and end of sleep than at the
beginning suggesting that the SP episodes at the later times might arise from brief
microarousals during REM, possibly induced by apnea. Reported frequency of SP
was also greater among those consistently reporting episodes at the beginning
and middle of sleep than among those reporting episodes when waking up at the
end of sleep. The effects of position and timing of SP on the nature of
hallucinations that accompany SP were also examined. Modest effects were found for SP
timing, but not body position, and the reported intensity of hallucinations
and fear during SP. Thus, body position and timing of SP episodes appear to
affect both the incidence and, to a lesser extent, the quality of the SP
experience.

PMID: 12028482 [PubMed - indexed for MEDLINE]