Three reasons why 'morning glory' exists, according to science

If you have a penis, chances are you've experienced nocturnal penile tumescence - or morning glory, as it's more commonly known.

In fact, the phenomenon is incredibly common and often happens around three to five times during an eight-hour period of sleep.

The good news is that if you do wake up with an erection, it means your junk is healthy. But the question remains - why?

Well, it turns out there are a few scientific explanations as to the cause of morning wood, the first being down to a person's sleep cycle.

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We'll let Sergio Diez Alvarez, director of medicine at The Maitland and Kurri Kurri Hospital and the University of Newcastle, explain...

"Nocturnal penile erections occur during Rapid Eye Movement (REM) sleep (the phase during which we dream)," he wrote in a post on The Conversation. "They occur when certain areas of the brain are activated."

Alvarez went on to say: "Sleep is made up of several cycles of REM and non-REM (deep) sleep.

"During REM sleep, there is a shift in the dominant system that’s activated.

"We move from sympathetic (fight and flight) stimulation to parasympathetic (rest and digest) stimulation."

The medical expert said that this shift in balance drives a response from the parasympathetic nerve system - and this causes that tent to get pitched.

The explanation makes sense, as people often wake up while coming out of REM sleep.

Another reason, said Alvarez, is testosterone levels.

"Testosterone, which is at its highest level in the morning, has also been shown to enhance the frequency of nocturnal erections," he said.

"Interestingly, testosterone has not been found to greatly impact visual erotic stimuli or fantasy-induced erections.

"These are predominantly driven by the 'reward system' of the brain which secretes dopamine."

Last but definitely not least, some professionals think it could be the body's way of letting you know you need to pee - which would explain why the tent goes down the minute dudes empty their bladder.

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"It’s more likely the reason for the morning erection is that the unconscious sensation of the full bladder stimulates nerves that go to the spine and these respond directly by generating an erection (a spinal reflex)," added Alvarez.

While scientific studies haven't fully concluded that nocturnal penile tumescences contribute to the health of a person's bits, some studies have suggested that if they stop abruptly, it could be a marker of diseases such as diabetes which impact erectile function, or severe depression.

Most of the time it's nothing to worry about, but if you notice any changes with those sturdy wake-up calls or you're concerned, it's always best to seek the advice of your GP or a medical professional.