Waking up with an erection is something all guys experience. And if the topic has ever come up (pardon the pun) with you and your buddies, you’ve probably heard different opinions why you get morning erections. A popular one is because of pressure on the penis from a full bladder (how many times have you also woken up with an urgent need to take a whiz?). Or maybe you’ve been told it’s because you’ve had an erotic dream… even if you swear you can’t recall any details.

In fact, neither of these explanations is true.

Scientists refer to morning erections as sleep-related erections (SREs). Rather than being associated with sex or bladder fullness, SREs are actually the body’s way of giving the penis a workout… no joke!

Mechanics of an Erection

Anatomical

There are two round, sponge-like structures called corpora cavernosa that run along the length of the penis. Through the center of each structure is an artery that allows blood to flow in. On the outside of the corpora cavernosa is a network of veins that lets the blood flow back out.

 

Did You Know?
Arteries pump oxygenated blood while veins pump deoxygenated blood.

An erection occurs when chemical messengers (neurotransmitters) cause the smooth muscles in the corpora cavernosa to relax. The two central arteries then open up, blood rushes into the penis and the spongy tissue expands. As a result, the outer veins are compressed and blood is prevented from flowing out of the penis. With the accumulation of blood, the penis stiffens and becomes erect.

 

Did You Know?
An erection can increase the size of the penis from 20 to 200 percent.

There is a second structure that surrounds the urethra (the tube running from the bladder to the tip of the penis) – the corpora spongiosum. It also fills with blood, but not enough to squeeze the urethra shut. This is why males can still take a leak even though they have an erection (but it does require some effort to aim accurately!).

Types

Guys experience 3 types of erections:

Reflexogenic – caused by tactile stimulation (e.g. masturbation) Psychogenic – due to visual associations (e.g. erotic or sexual images) Nocturnalthe subject of this week’s question

Nocturnal Erections

When we’re sleeping, we pass through five stages: 1, 2, 3, 4 and REM (rapid eye movement) sleep. These stages are cyclical - after going from 1 through REM, we begin again with stage 1.

 

Did You Know?
A complete sleep cycle takes an average of 90 to 110 minutes.

It is during REM sleep that males develop nocturnal erections. These result from androgens (male sex hormones such as testosterone) triggering the production of nitric oxide (NO). NO is a neurotransmitter that stimulates the flow of blood to the penis. It appears that these reactions are subconsciously initiated in the brain.

 

Did You Know?
Research suggests that androgens have little or no influence on erections caused by tactile or visual stimulation.

Guys start experiencing nocturnal erections at an early age, but the frequency increases once they reach puberty, when testosterone levels rise. Later in life, the frequency drops off again.

 

Did You Know?
Nocturnal erections typically occur three to five times during the night, each lasting about 20 minutes.

The main purpose of nocturnal erections is to periodically provide the corpora cavernosa with a supply of fresh blood and oxygen. If this didn’t happen, there is a risk that that the corpora cavernosa may lose its ability to prevent the outflow of blood from the penis during an erection – a condition known as corporal veno-occlusive dysfunction, the most common cause of erectile dysfunction (ED) in men.

In closing…

So as we can see, waking up with an erection in the morning is actually a good thing. It’s a way of letting guys know that, while they’ve been sleeping, their penis has been faithfully “exercising” to stay in shape and remain healthy.

Learn More!

Erections and wet dreams

Using nocturnal erections to test for erectile dysfunction

Hirshkowitza, M, and M.H. Schmidt. 2005. Sleep-related erections: Clinical perspectives and neural mechanisms. Sleep Medicine Reviews 9, 311–329.

Montorsi, F. and M. Oettel. 2005. Testosterone and Sleep-Related Erections: An Overview. Journal of Sexual Medicine 2, 771-784.

CRAM Science would like to thank Dr. Stan Megraw for his help and expertise in answering this question. Stan is a writer/researcher specializing in science, technology and medicine. He has more than 30 years experience as contributing author for research institutes, universities, government agencies and non-profit organizations.

Stan Megraw

Stan is a writer/researcher, a PhD graduate of McGill University and was a member of the CurioCity team for several years. As a kid he dreamed of playing hockey in the NHL then becoming an astronaut with NASA. Instead, he ended up as an environmental research scientist. In his spare time Stan enjoys working on DIY projects, cooking and exploring his Irish roots.


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