Simplify the following text for a Grade 8 native English-speaking person.
Original: In some males , erection can occur spontaneously at any time of day , and is known as nocturnal penile tumescence when occurring during rapid eye movement sleep .
Simplified: 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 .