Nocturnal penile tumescence (NPT) refers to erections that occur during sleep. These erections are normal and happen nightly through a man’s life. But they are not caused by sexual stimulation.
Typically, men have 3 to 5 erections during 8 hours of sleep. A single erection may last for 30 minutes or longer.
Nocturnal erections seem to follow a man’s sleep cycles.
When people sleep, they alternate between two different kinds of sleep, which are distinguished by eye movement. During the rapid eye movement (REM) cycle, the eyes move, and dreams may occur. During non-REM sleep, the eyes stay still.
Sleep always begins with a non-REM cycle, followed by an REM cycle. The two types switch back and forth throughout the night.
NPT occurs during REM sleep, when the brain produces smaller amounts of a neurotransmitter called norepinephrine, which is involved with erections.
When a man has an erection, his penis fills with blood. Norepinephrine actually prevents erections by stopping the flow of blood into the penis.
But during REM sleep, when norepinephrine levels drop, more blood can flow into the penis, causing nocturnal erections.
This process also explains why men often wake up with erections. “Morning wood” happens when a man wakes up during or just after REM sleep.
While the exact reasons for these erections are not clear, they are thought to be one way a man’s body keeps the penis healthy.