Penile tumescence is a phenomenon believed to be closely related to the REM cycle of sleep. Erections linked to REM occur several times during sleep but are known as morning erections when they occur upon waking.Continue Reading
The REM cycle is the deepest sleep, which is characterized by intense brain activity. Dreams occur during this cycle, and they can sexually arouse the body. Sexuality expert Cory Silverberg also suggests increased hormone levels and heart rate as possible causes but explains that penile tumescence is not fully understood and requires more research.
Waking without an erection, coupled with difficulty achieving an erection while awake, can point to erectile dysfunction.Learn more about Human Anatomy