Men often wake up with an erection first thing in the morning, but what causes morning wood or morning erections? Learn about this phenomenon, why it is associated with REM sleep, and what it might mean if it doesn't occur.

Verywell / Brianna Gilmartin

Morning Erections and the Role of Sleep Stage

Morning erections might actually relate to the timing of rapid eye movement (REM) sleep. As part of normal sleep patterns, called sleep architecture, transitions occur throughout the night between the various sleep stages. As morning approaches, the REM period becomes prolonged. The last third of the night may be predominated by REM sleep. Therefore, when you first wake up, you are often coming out of REM sleep.

In addition to the vivid movie-like dreams that are characteristic of REM sleep, the penis also becomes erect and engorged with blood during these periods. This is due to the activity of the parasympathetic nervous system affecting the sacral nerve. This is technically called penile tumescence. When this occurs in the morning, the stiffened penis may be referred to in slang terms as "morning wood" or "morning glory."

Therefore, morning erections are secondary to the timing of REM sleep and the natural physiological processes that are associated with this sleep state in men. It seems that the body is testing systems during sleep, and this is a normal and healthy phenomenon in men. It may even be noted in young children. It is also possible for the clitoris of a woman to become engorged with blood and erect during REM sleep.

These erections typically go away within a few minutes after awakening. A persisting erection, when the blood fails to drain from a hard penis, may occur in a condition called priapism.﻿﻿﻿A lack of morning erections may suggest a problem with erectile dysfunction.

The Normal Timing of Erections in Sleep

REM sleep periods occur starting 80 to 110 minutes into sleep and may occur at this interval periodically three to four times throughout the sleep period.﻿﻿ As noted, REM sleep becomes more prolonged towards morning. Therefore, more REM sleep and more erections occur towards morning.

The precise timing of these erections may be difficult to predict as there is quite a bit of variability. The length of time from the onset of the erection, with a 2 mm change in the circumference of the penis, to a full erection is usually 10 minutes. The erection usually lasts 25 minutes on average.

Though not routinely performed, the erectile function of the penis can be tested as part of a diagnostic sleep study called a polysomnogram.

What If Morning Erections Do Not Occur?

In general, sleep-related erections decrease with age. This may correlate with impotence that occurs during the daytime. In contrast, the presence of erections in sleep when daytime impotence occurs may suggest non-physiological causes of the difficulty maintaining an erection (such as stress or performance anxiety). Interestingly, untreated obstructive sleep apnea may also decrease erections.

If you do not wake with an erection, it is not necessarily abnormal. It may be that you did not wake from REM sleep. It is also possible that the erection began to fade prior to waking completely.

No matter the cause, if you have problems maintaining an erection, speak with your doctor about treatment options. For those men who frequently wake with a morning erection, rest assured that this is a normal part of sleep.