Rather than being caused by sexual arousal, nocturnal erections are caused by regular functions of the male reproductive system.

In REM sleep, the deepest sleep stage in the cycle, the body’s parasympathetic nervous system is activated. This system is responsible for rest and relaxation, opposed to the sympathetic nervous system, which is responsible for your fight or flight response. The parasympathetic nervous system is also activated during sexual arousal.

While men are in REM sleep they can experience spikes in hormones, which may be a trigger for nocturnal erections. Together, the parasympathetic nervous system, the male reproductive system, and REM sleep all affect a man’s likelihood to have nocturnal erections.

Not all men experience regular morning erections though. It’s just as normal to experience one every day as it is to experience it just once or twice a week.