Science Has No Definite Explanation For Morning Wood. The clinical term is "nocturnal penile tumescence." The old "it keeps you from peeing in bed" explanation we've all heard isn't completely true. The theory is plausible, but scientists can't explain why BEMHO (look it up on Urban Dictionary, grandma) comes and goes 3-4 times a night. There is actually no rock-solid explanation for why it occurs. Sorry.