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Bedtime erections are everyone's business, or that's what historical treatment of the nocturnal phenomenon suggests, according to a 2014 paper by Mels F. van Driel, a urologist at the University of Groningen in The Netherlands. Since early Christianity, philosophers, theologians, urologists, sexologists, other types of doctors, psychologists and church courts have weighed in on erections that spring up after people turn in. Today we know that sleep-related erections (we'll call them SREs) are a normal, non-sexual occurrence associated with REM sleep, but we still don't understand exactly why they happen or which brain regions issue the call to attention. 

The practice of characterizing the penis as an animate creature rather than a bodily organ isn't exclusive to modern times. Apparently, Plato assigned agency (and masculine pronouns) to the appendage back around 300 BC. The ancient Greek thoughtleader said that "the penis possessed his own will," and that "to keep him under control is a complex and skillful exercise in which not everybody will succeed." A few hundred years later, the Roman poet Horace chronicled his experience waiting in bed until midnight for a girl, before falling asleep "with my penis completely erected, and my dreams stain my night-short and my supine belly with obscene images."

We still don't understand exactly why they happen or which brain regions issue the call to attention.

By the first century, medical minds were chiming in on SREs. Claudius Galenus, personal physician to the Greco-Roman emperor Marcus Aurelius, saw a person's dreams as a manifestation of their physical state. When men were full of sperm, Galenus held, they had erotic dreams and, as a result, raised quilts. Galenus was wrong about the sexual nature of SREs, but right about their timing. Centuries later, sex researchers would figure out that SREs pop up in a cyclical pattern corresponding to REM sleep.

Plato-Statue

Religious thinking seems to have driven understanding of SREs in the pre-medieval era. Morning wood, (chronologically, the last erection to count as an SRE) erased the God-points earned through bedtime prayer. To prevent SREs, deemed "attacks," monks would hit the sack with metal crucifixes fastened to their penises. But, not every religious authority saw SREs as sinful. St. Augustine, for example, said men couldn't be held responsible for what they did while dreaming. "So, for Christian monks," van Driel wrote, "SREs and nocturnal ejaculations became the battlefield to preserve their purity." But, starting in the 14th century, men would get in trouble for not having SREs.

The Impotence Trials

The Catholic Church scorned married men with erectile dysfunction. In fact, limp genitalia was grounds for divorce under ecclesiastical law. Through 1677, Impotence Trials investigated penile-performance allegations in an increasingly invasive manner. Defendants had to prove their penises worked, which meant sleeping in the presence of jury members who'd verify or deny the occurrence of SREs. By the 16th century, erection-watch parties got even weirder, requiring "a demonstration of the rigid erection but also of its elasticity and natural movement," wrote van Driel. Sometimes, defendants had to demonstrate ejaculation and, eventually, were even having sex before jurors' eyes. "Undoubtedly," van Driel wrote, "the obsessive discussions about erectile dysfunction and other sexual abnormalities served as an outlet for suppressed sexual feelings."

Defendants had to prove their penises worked, which meant sleeping in the presence of jury members who'd verify or deny the occurrence of sleep-related erections.

The early 1800s saw men of medicine invent tools and contraptions to prevent SREs, including spiked penis rings, a torso-length penis corset, "a metal tube that dangled from a leather jerkin," and partitioned beds that let boarding-school monitors catch "suspicious movements." More serious erection analyses transpired towards the end of the century. Some theories missed the mark, such as one scientist's belief that a full bladder caused morning erections. But other early researchers succeeded in overturning penile misconceptions. By keeping a detailed account of his own SREs, the scientist Henry Havelock Ellis figured out that ejaculations decrease with age, contrary to popular belief.

Science Starts Perking Up

Formal SRE studies really began in the 20th century, by which point psychiatrists and sex researchers acknowledged SREs as normal for boys and men of all ages. In the 1930s, German scientists monitored SREs in baby boys, nine "three-to-20-week-old completely undressed male infants with diapers spread out smoothly beneath their buttocks," who frequently woke up when SREs occurred. A decade later, different Germans published a two-part study in which they measured SREs during nighttime and daytime rest. They found that erections occurred every 85 minutes, on average, and lasted for about 25 minutes. They, too, found a strong link between spontaneous awakenings and the observed erection cycle.

By the 16th century, erection-watch parties got even weirder, requiring "a demonstration of the rigid erection but also of its elasticity and natural movement,"

A few years later, US researchers realized that erection cycles synced up with REM sleep. Ivan Aserinsky and Nathaniel Kleitman noted the pattern in 1955, two years after the duo discovered REM sleep at the University of Chicago. Then, in 1965, the New York brain physiologist Charles Fisher devised three experiments to study SREs in action. In the first method, Fisher placed a donut-shaped tube, filled with water, around the base of the penis. The second method measured penile-skin temperature. And the third involved a silicone tube and a mercury gauge. All three methods shared the same flaw: The devices (one of which was speculated to be an artificial vagina) might themselves cause erections through penile stimulation. How could Fisher reliably distinguish between bulges of arousal and true SREs? So, Fisher and colleagues took a back-to-basics approach, and monitored penile movement in men who slept underneath clear sheets, free of clothing and genital contraptions. 

The Heyday of Hard-on Research 

Plainly observing and rating the intensity of SREs, explained van Driel, was also the method of choice in the '60s and '70s for Ismet Karacan, the true pioneer of SRE research. Through lab experiments, the psychiatrist deemed SREs unrelated to erotic dreams. And, as men get older, Karacan found, they have fewer and shorter SREs, which happen later in the sleep cycle and may occur outside REM sleep (a rarity among youngins).

Karacan also floated the idea of using SREs to diagnose erectile dysfunction as either psychological or physiological. While he emphasized the value of performing tests in a sleep lab, more cost-effective methods took over in the '80s. The popular stamp test, for example, involved wrapping a roll of stamps around the penis before bedtime. If the roll ripped overnight, then SREs had gone down (and up), suggesting erectile problems were mental. If the roll remained in-tact, then the dysfunction was presumed to be physical. 

"For Christian monks, sleep-related erections and nocturnal ejaculations became the battlefield to preserve their purity."

The mental-versus-physical debate has died down, said van Driel, as most doctors now see the difference as irrelevant. But this shouldn't be a revelation. Researchers had noted that psychological and lifestyle factors adversely affect SREs decades earlier. One big variable? Sleep, and the lack of it. In the '60s, German researcher Uros Jovanovic said sleep quality and slumber wood went hand in hand, writing: "Good Sleepers and Good dreamers have the best SREs, while poor sleepers but good dreamers take the second place. Good sleepers and weak dreamers have the third place; the weakest SREs are observed in poor sleepers and poor dreamers."

What We Know Today

Through neuroimaging tools, sleep tests (i.e., polysomnography, or PSG) and other physiological measures, experts have learned that SREs peak (in duration and intensity) during puberty. Teen boys spend about 30 percent of the night with SREs, a figure that falls by 20 percent for men in their 60s. But, the purpose and neurophysiological roots of SREs are still up for debate. Leading sexologists, sleep specialistis, urologists and psychiatrists have theories, but no definitive answers. In the '90s, Harvard researchers proposed that SREs improve oxygen flow to penile tissue and help maintain normal erectile responses. Basically, low oxygen availability might contribute to erectile dysfunction, an idea supported by research on mountain climbers.

The brain basis of bedtime boners is another doozy. According to the theory of context-specific erectile dysfunction, van Driel wrote, "it may be possible that several different areas of the brian contribute to the occurrence of erections in different contexts." This would mean that separate "specialized higher central structures" are responsible for different types of erections, such as SREs, erections caused by sensory stimulation (touch, smell, sight) and "mental" erections sparked by memory or fantasy. The journey has been a long and hard one, but we've gotten there.