Penis tumescence
Penis tumescence is an erection which occurs during sleep. These erections in the night are very common between men and happen throughout the man’s life. But these erections do not occur due to sexual excitement .
The sleep of people is alternated between two types, which are divided by the eye movement. One is rapid eye movement (REM) in which human eyes move, and dreams may happen. Another is non rapid eye movement in which eyes remain still.
Human sleep always start with a non rapid eye movement cycle and followed by a rapid movement cycle. These two types of cycle switch back and forth throughout the night.
Men penis fills with blood, when they get an erection. Norepinephrine stops the blood flow into the penis, which actually prevents erection.
But when norepinephrine level drops during rapid eye movement sleep, more blood flow leads to nocturnal penile tumescence erection.