Nocturnal penile tumescence is believed to contribute to penile health. Erections during sleep or upon waking up are known as nocturnal penile tumescence (NPT). The cause of nocturnal penile tumescence is not known with certainty. During erection, the corpora cavernosa will become engorged with arterial blood, a process called "tumescence". Along with nocturnal clitoral tumescence, it is also known as sleep-related erection. Tumescence is the quality or state of being tumescent or swollen. These medications are all types of vasodilators and cause tumescence within 10-15 minutes.