Oasis, Morning Glory neon sign, Berwick Street, SOHO, London, England, UK

Nocturnal penile tumescence is a spontaneous erection of the penis during sleep or when waking up. Along with nocturnal clitoral tumescence, it is also known as sleep-related erection, morning glory or morning wood. Men without physiological erectile dysfunction or severe depression experience nocturnal penile tumescence, usually three to five times during a period of sleep, typically during rapid eye movement sleep. Nocturnal penile tumescence is believed to contribute to penile health The cause of nocturnal penile tumescence 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 tumescence. Bancroft (2005) hypothesizes that the noradrenergic neurons of the locus ceruleus in the brain are perpetually inhibitory to penile erection, and that the cessation of their discharge that occurs during rapid eye movement sleep may allow testosterone-related excitatory actions to manifest as nocturnal penile tumescence. 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 one’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