The cause of<unk>  is not known with certainty. In a wakeful state, in the presence of mechanical stimulation with or without an arousal, erection is initiated by the parasympathetic division of the autonomic nervous system with minimal input from the central nervous system. Parasympathetic branches extend from the sacral plexus of the spinal nerves into the arteries supplying the erectile tissue; upon stimulation, these nerve branches release acetylcholine, which in turn causes release of nitric oxide from endothelial cells in the trabecular arteries, that eventually causes<unk> . Bancroft (2005) hypothesizes that the noradrenergic neurons of the locus ceruleus in the brain are perpetually inhibitory to<unk>  erection, and that the cessation of their discharge that occurs during rapid eye movement sleep may allow testosterone-related excitatory actions to manifest as<unk> . Suh et al. (2003) recognizes that in particular the spinal regulation of the cervical cord is critical for nocturnal erectile activity. The nerves that control a man’s ability to have a reflex erection are located in the sacral nerves (S2-S4) of the spinal cord. Evidence supporting the possibility that a full bladder can stimulate an erection has existed for some time and is characterized as a 'reflex erection'. A full bladder is known to mildly stimulate nerves in the same region. The possibility of a full bladder causing an erection, especially during sleep, is perhaps further supported by the beneficial physiological effect of an erection inhibiting urination, thereby helping to avoid nocturnal enuresis . However, given females have a similar phenomenon called nocturnal clitoral<unk> , prevention of nocturnal enuresis (bed-wetting) is not likely a sole supporting cause. In an old study, during puberty, the average<unk>  time per night was 159 min; average REM sleep time was 137 min. Average simultaneous REM sleep and<unk><unk>  per night was 102 min. Study subjects averaged 6.85<unk>  episodes/night, and, of these, 5.15 occurred during a REM sleep period.<unk>  episodes during REM averaged 30.8 min in duration, whereas episodes which occurred when no REM was present averaged 11.75 min. Study subjects had at least four REM periods per night and at least three<unk>  episodes. In another old study of healthy aging men, frequency and duration of<unk>  decreased progressively with age independent of variations in sleep. In contrast to men in the younger age groups, the majority of men above age 60 did not have full sleep erections even though they and their partners reported regular intercourse. Unlike physiological<unk><unk> , sleep-related painful erections (SRPE) and Stuttering priapism (SP) are much rarer pathological erections, resulting in poor sleep and daytime tiredness, and long term cardiovascular morbidity. SRPE, though also occur predominantly during REM sleep, without an apparent underlying illness are painful and thus interrupt sleep. On the contrary, stuttering priapism can occur spontaneously at any time of the day, but more commonly so during REM sleep. SP is a subtype of ischemic priapism that is characterized by recurrent, self-limiting, painful erections that often require maneuvers (compression, cold packs or a cold shower, voiding, or exercise, etc.) to aid detumescence. In ischemic priapism, most of the penis is hard; however, the glans penis is not. Much rarer priapism is secondary to blunt trauma to the perineum or penis, with laceration of the cavernous artery, which can generate an arterial-lacunar fistula resulting in a high blood flow state, hence the<unk> .<unk>  lasting for more than four hours is a medical emergency.
Nocturnal penile tumescence
Nocturnal clitoral tumescence
Premenstrual water retention
Nocturnal enuresis
Morning pseudoneutropenia
Task-invoked pupillary response
Clitoral erection
Post-void dribbling