nocturnal penile tumescence (NPT)
: spontaneous erection of the penis during sleep occurring from birth to advanced old age, typically in three episodes a night for a total of 2-3 hours. It is associated with the REM (rapid eye movement) phase of sleep and with erotosexual dreams. It is measured by harnessing the penis into an expandable ring. Less is known about the corresponding phenomenon in females as there is no fully satisfactory technique for measuring vasocongestion and lubrication of the female genitals.
Dictionary of Sexology Project: Main Index