Why Do Healthy Men Experience Morning Erections ?

People begin the sleep cycle with a period of non-rapid eye movement (NREM) sleep, followed by a very short period of rapid eye movement (REM) sleep. During a normal night of sleep, humans usually experience about four or five periods of REM sleep. Penile erections have generally been considered to be an epiphenomenon of REM sleep-related physiological changes for healthy males. Thus, men are very likely to awaken in the morning with a REM sleep-related erection, which is also known as nocturnal penile tumescence (NPT). Men who are physically under great strain or serious psychological stress may find it difficult to maintain a psychogenic erection. The best time for them to have sex would be during sleep time, such as when they are experiencing REM sleep-related erections. It is reasonable to assume that the NPT phenomena might have evolved as a tool for having sex, in the context of both procreation and recreation.


NOCTURNAL PENILE TUMESCENCE Penile Erections During Sleep
Spontaneous awakening from sleep is significantly associated with the erection cycle [5] and the association had already documented in the early 1940s [15]. Sleep is different for each individual, but most men have four to five full erections during REM sleep, with each erection lasting about 25-35 minutes. The erections are a normal part of REM sleep for males of all ages, even in infants and children. The relationship between REM sleep and erections is the reason that men are likely to awaken directly from REM sleep, or soon after a long morning REM period has ended. Thus, even though men do not always wake up at the end of each episode of REM sleep, they are very likely to awaken in the morning with an REM sleep-related erection [5,16]. Morning erections are scientifically defined as involuntary sleeprelated erections (SREs) or nocturnal penile tumescence (NPT) and are a healthy and normal physiological response that most men experience [5, 16 -19].
Erections are considered a normal part of sleep physiology in men. Evaluation of NPT is one of the earlier tests devised to study erectile dysfunction (ED). After demonstrating that NPT was a general phenomenon in healthy males 3 to 79 years old, Karacan, in the early 1970s, suggested that NPT could be used to evaluate ED [4,20]. The mechanism of NPT is presumed to rely on neurovascular response mechanisms similar to those seen in erotically induced erections. Thus, men who are documented to have normal NPT are presumed to have a normal capacity for spontaneous, erotically induced erections. As a matter of fact, the primary function of NPT tests is to distinguish psychogenic causes of ED from organic causes [18, 21 -23]. Men who do not have erections because of psychological problems can still have erections during deep sleep. If a lack of morning erections is accompanied by a lack of overall erections, a physical cause is suggested rather than a psychological cause [21,23,24].

Physiology of NPT
Although it has been known for some time that every REM cycle is associated with penile erections in males and clitoral engorgement in females, it is not known why this sexual excitation occurs. The erections are not related to erotic content of dreams, previous sexual activity, or a full or empty bladder [11]. Oddly enough, the erections have generally been considered as mere epiphenomena of REM sleep-related physiologic changes [3,11,16]. That is, morning erections have been connected to some hormones and neurotransmitters.
Sex-related hormones have been selectively associated with REM sleep. Testosterone modulates nearly every component involved in erectile function, from pelvic ganglions to smooth muscle and the endothelial cells of the corpora cavernosa. Testosterone also regulates the timing of the erectile process as a function of sexual desire, coordinating penile erection with sex [25 -27]. Testosterone levels are higher early in the morning compared to the afternoon or evening in males. In general, the levels rise on falling asleep, are highest at the transition from non-REM to REM sleep state, peak around the time of the first REM sleep period, and remain constant until awakening. Testosterone has been found to be the cause of NPT [11, 17, 18, 27 -29]. Prolactin is also known to be crucial for the development of reproductive and sexual behaviors. It stimulates an array of testicular functions in males and ovarian functions in females [30]. Its release is dependent on REM sleep, with its level rising rapidly at sleep onset and peaking around 3-5 a.m. when REM sleep dominates [11,30].
Penile erection is neurologically controlled by the autonomic nervous system. Several neurotransmitters, especially norepinephrine and acetylcholine, are well-known regulators of penile erection [19]. During REM sleep, high levels of acetylcholine in the hippocampus suppress feedback from the hippocampus to the neocortex, and lower levels of acetylcholine and norepinephrine in the neocortex encourage the spread of associational activity within neocortical areas, without control from the hippocampus [31,32]. This leads to the cessation of histamine, norepinephrine, and serotonin neuron activity during REM sleep [3,6]. The erections in REM sleep occur when neurological stimulators cause blood to flow into the penis. This ensures that oxygen is properly delivered to the penis and helps keep penile tissue healthy [2,33].

Functions of NPT
What are fundamental functions of NPT? It is reported that REM sleep is present during a considerably higher percentage of the sleeping time of infants and children than adults. Since penile erections tend to be synchronous with REM sleep, this again suggests that the erections may not be primarily a sexual phenomenon, at least to infants and children [34]. There are two types of erections. One is a psychogenic erection which occurs due to the stimulation of some parts of the brain, and the other is reflex erection which may spontaneously appear at any time, such as NPT [35]. A psychogenic erection is triggered by sexual stimulation and sexual arousal [19].
The major functions of sexual interaction are both procreation and recreation [36]. A man should maintain either a psychogenic or reflex erection for penile-vaginal intercourse. If he wants sex with a psychogenic erection, he should be awake and sexually aroused by the erotic and/or emotional stimuli. He could, however, have sex with an REM sleeprelated erection, even without stimuli. Here, it is possible to surmise the reason why men experience REM sleep-related erections spontaneously, in the context of procreation. The brain of every man instinctively seeks to eternal generation. On the instinctive level, this leads to a man looking for a sexual partner to transfer his genes to create future generations [37]. This is the most important role played by erections, as they are one of the most necessary tools for sexual intercourse.

Sexual Intercourse with NPT
In general, men who are physically under great strain may have difficulty maintaining a psychogenic erection [38]. For instance, monogamous men, in prehistoric societies, might come back home with physically fatigued from gathering and/or hunting. If so, it might not be pertinent for them to have sexual intercourse immediately after coming back from work early in the evening. The best time for them to have sex would be after getting enough rest during sleep time, such as experiencing REM sleep. Thus, men's REM sleep-related erections during sleep might have evolved to allow procreation irrespective of their will or intention [11].
What about the men of today? The life of the modern men is full of stresses and constant strain, both physically and psychologically. If they work all of the time, take care of kids, and stay busy most of the day, it might not be easy to have sex before they fall asleep [39]. Also, there are many men who suffer from ED, owing to psychological strain [22,23]. Under psychologically serious stress, men become too anxious to get or maintain an erection, and thus, suffer from either the loss of erection and/or premature ejaculation [38]. For instance, men who have relationship problems with female partners do not attempt to have sex owing to anxiety [40]. They fear that when they initiate sex, their female partners might reject the initiation. Even though the female partners might not reject the initiation, the men would experience erectile problems and/or premature ejaculation [38].
Women experience clitoral enlargement, as men do penile erections, during the REM sleep state due to increased blood flow to genital organs [6,41]. This indicates that a woman's body is ready to have sex during the REM sleep state. It also indicates that women who are semi-awake would be less resistant to their partner, even if they are currently uncomfortable with their partner [42]. As mentioned earlier, some researchers stated that REM sleep evolved to promote attachment in unattached sexual partners [12,13]. That is, men who have relationship problems may try sexual intercourse by genital erections during REM sleep [14].
As for Korean couples, some who had relationship problems with their spouses do not choose divorce or separation as a solution. Many of them tend to believe that the problems can be solved if they have sex with their partners [42]. This means that they believe marital conflicts might be solved through sex, no matter the situation [42,43]. Many couples in marital conflict choose sleeping in separate rooms for a specific period of time (e.g., for a week, month, or even longer than that) as a solution [43,44]. While researching these couples, the authors met many women who reported having sex with their husbands during the separate room arrangement. Women, who refused their husbands at other times, did not reject their husbands who came to their room while they were sleeping, when they had penile erections [42 -44]. This indicates that their sexual encounters by the REM sleep-related erections would serve a palliative function especially during time of physical and/or psychological stress [14].
According to a study of part-time college students who had their sexual partners, some of them reported to have sex early in the morning before getting up. In general, the majority of sexual encounters take place at bedtime, that is, people begin to sleep after having sex, while some of the encounters do occur during the daytime [45]. However, approximately 20% of sexual encounters take place early in the morning [46]. It is reasonable to assume that REM sleep-related erections have a function of recreation, such as resolving marital conflict. According to Komsomolskaya Pravda, some Russian women said they had sex during the sleep. Many women who never experienced orgasm before are reported to have achieved peak of sexual satisfaction while having sex half-asleep [47].
Moreover, sex with an REM sleep-related erection could be advantageous to procreation. If a man has sex with his partner who is in a sleepy state, she may lie back for sex and not move her body much. It is more suitable for sperm to reach her cervix, and thus she can retain more sperm [48]. This means the chances of getting pregnant might be somewhat higher in sex with the REM sleep-related erections than other times. That is, if REM sleep might have compensatory and reproductive functions, then the REM sleep-related erections may exert a strong influence on the reproductive success that has greater evolutionary importance [11 -14].

CONCLUSION
Healthy men experience erections owing to physiological changes such as sex-related hormonal release during REM sleep. The hormone release leads to REM sleep-related erections. Men are able to have sex with these erections, which may have the benefits of enabling sexual excitation, couples resolving marital conflicts, and their partners getting involved [11,16,19,42,49]. In conclusion, it can be said that REM sleep promotes development of both attachable and/or recreational and reproductive strategies in human beings [11 -13], while REM sleep-related erection has been observed to be helpful for couples who are under stress physically or psychologically to have sex [13,14]. Accordingly, it is said that sex with an REM sleep-related erection would be the best opportunity for both procreation and recreation.

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