The scientific term for a wet dream is nocturnal emissions. A wet dream is when a penis gets erect (hard), and ejaculates (releases semen) while a person is asleep. A sign that a nocturnal emission has happened is waking up in damp pyjamas and bed sheets. Nocturnal emissions usually begin during puberty as the body produces more testosterone. Some people experience them and some may find them embarrassing, but they are completely normal. Wet dreams are one sign puberty has started.