Erections of the penis ( nocturnal penile tumescence or NPT) normally accompany REM sleep in rats and humans.  If a male has erectile dysfunction (ED) while awake, but has NPT episodes during REM, it would suggest that the ED is from a psychological rather than a physiological cause. In females, erection of the clitoris ( nocturnal clitoral tumescence or NCT) causes enlargement, with accompanying vaginal blood flow and transudation (. lubrication). During a normal night of sleep the penis and clitoris may be erect for a total time of from one hour to as long as three and a half hours during REM.