What is "nocturnal penile tumescence"

alt. (context euphemistic English) An erection during sleep or on waking up. n. (context euphemistic English) An erection during sleep or on waking up.

Nocturnal penile tumescence (abbreviated as NPT), also known colloquially as morning wood or morning glory, is a spontaneous erection of the penis during sleep or when waking up. All men without physiological erectile dysfunction experience nocturnal penile tumescence, usually three to five times during the night, typically during REM sleep. NPTs are believed to contribute to penile health.