This answer is a summary of the entry "Why do men wake up in the morning with an erection?" from the book "Why Do Men Have Nipples?" by Mark Leyner and Billy Goldberg, MD. . . Erections (more scientifically called nocturnal tumescence) occur during REM sleep and are spontaneous. Erections can sometimes happen spontaneously without any stimulation, and morning erections are examples of this phenomena.. . Hope this is helpful! :-)