REM Latency (Last Updated - 6/25/2003)
-time from the moment you fall asleep to the the first REM period
-not from the time you goto bed but from the time you fall asleep
-typically ~90mins
-D/O's which shorten REM Latency:
   1) Depression (MDD) (shortens to ~60mins)
   2) Narcolepsy (shortens to ~10mins)

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References:

Kaplan USMLE Step 2 Lecture Notes: Psychiatry and Epidemiology & Ethics. Gonzalez-Mayo A. 2002. pg. 79.