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Why do men wake up in the morning with an erection?

Just curious. It's kind of funny. :)

3967 day(s) ago

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Most men, at some time or another, wake up in the morning with an erection. The frequency in which this happens varies by individual. Colloquially, this phenomenon is known as morning wood. Scientifically, it is called nocturnal penile tumescence, or NPT. NPT can occur at any time during the night, not only just before waking.

The exact cause of NPT is still unknown to medical science. One researcher theorizes that NPT has to do with a type of nerve called a noradrenergic neuron. The noradrenergic neurons in the nucleus of the brain stem are responsible for making a penis go flaccid. This doctor says that these neurons are always at work whenever a man has a flaccid penis. When sexually aroused, these neurons stop sending their flaccid signals so men can get an erection. During REM sleep, the neurons stop firing and the male hormone, testosterone, begins to excite the nerves in the penis, causing a spontaneous erection during sleep.

Another theory states that when the bladder fills with urine it stimulates nerves in the S2-S4 region of the spinal cord. This region is the same one used by the brain stem to send signals to the penis causing it to be either erect or flaccid. During the day, men have too many other stimuli all competing at the same time in the body, so the stimulation by a full bladder is not given any priority to be felt. When sleeping, however, outside stimuli are minimal, so the full bladder stimulation has a pathway to enact. Those who support the full bladder theory also note that having an erection prevents the bladder from discharging, so the body may be doing this by design to prevent urination while sleeping.

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