An erection of the penis can also happen by itself when a man is asleep, often when he is dreaming. The scientific term for this is "nocturnal penile tumescence" (which means "swelling of the penis at night"). It is also common for a man to wake up after a night's sleep to find that he has an erection. This is often caused by his bladder being full of urine, which presses on the tissues in the penis. Such an erection is sometimes informally called a "morning glory" or "morning wood".