Most men wake up with an erection at least some of the time, and it usually has nothing to do with a sexy dream. Through the night your brain cycles through stages of sleep, and during REM (rapid eye movement) sleep, the stage most linked with vivid dreaming, the penis becomes erect on its own. Researchers call these sleep-related erections or nocturnal penile tumescence (NPT). Morning wood is simply the one you happen to wake up during.
Sleep-related erections occur in all healthy men with normal erectile function, closely tied to REM sleep.1 In a laboratory study of healthy men, the vast majority of night-time erections were coupled to REM episodes, although the exact timing between the start of REM and the start of an erection varied a lot.2
Nobody knows exactly what these erections are for. They involve blood vessels, nerves and hormones working together, and the night-time cycle seems to be largely unaffected by what you did during the day.1
Why do men get morning wood?
Morning wood is a by-product of REM sleep. Erections switch on during REM phases throughout the night, and if you wake during or just after one, you notice it.
The erections arrive in step with REM sleep across the whole night, not just in the morning; you simply aren't awake for most of them.12 A full bladder is a popular explanation, but the association researchers describe is with sleep stage.
Whether you notice morning wood depends partly on when you surface. Waking from a different part of the sleep cycle, or sleeping badly, can mean you miss it on a given day. For more on how rest shapes sexual health, see sleep and sex drive.
Does morning wood decrease with age?
A little. Sleep-related erections continue across the lifespan with only a slight decline in healthy older men, so a complete or sudden loss is worth mentioning to a doctor.
A classic study that monitored 125 healthy males aged 3 to 79 with overnight brain-wave recordings found that night-time erections occurred consistently, but that their pattern was significantly affected by age.3 A later review described the decline in healthy older men as slight.1
Hormones matter too. In the European Male Ageing Study of 3,369 men aged 40 to 79, poor morning erections were one of three sexual symptoms most closely tied to low testosterone, alongside low sexual desire and erectile dysfunction.4
What does it mean if I don't get morning wood?
Missing it now and then means little. A lasting change, especially alongside low desire or erection problems during sex, is worth a GP visit and possibly a testosterone test.
Sleep-related erections have long been used clinically as an objective way to judge whether the physical capacity for an erection is intact, and to help tell physical causes of erectile dysfunction from psychological ones.13 Formal testing uses overnight monitoring, ideally in a sleep laboratory, rather than what you happen to notice on waking, so treat your own observations as a rough guide only.1
Still, the pattern can be a useful thing to mention. If morning erections have faded along with your sex drive and erection quality, that combination of three sexual symptoms is the one most closely linked with low testosterone and hypogonadism.4 A blood test can check. If you still wake with firm erections but struggle with a partner, performance anxiety is worth reading about.
Common questions
Is it normal to not have morning wood every day?
Yes. Whether you wake during or just after a REM phase varies from night to night, so the pattern over several weeks tells you more than any single morning. A gradual, lasting change is what's worth noting.
Is morning wood a sign of high testosterone?
Not exactly. It's a sign that the erection system works, and testosterone is one of its inputs. In a large European study, poor morning erections were linked with low testosterone, but having them doesn't tell you your levels are high.4
Do women have something similar?
Yes. Sleep researchers describe analogous night-time genital changes in women.1 They're just less visible, so they don't have a nickname.
Read more on this
Morning wood explained: what it does (and doesn’t) tell you about your health
Sleep and sex drive: the overlooked connection
Sources
- Hirshkowitz M, Moore CA. Sleep-related erectile activity. Neurol Clin. 1996;14(4):721-37. PubMed
- Mann K, et al. Temporal relationship between nocturnal erections and rapid eye movement episodes in healthy men. Neuropsychobiology. 2003;47(2):109-14. PubMed
- Karacan I, et al. Sleep-related penile tumescence as a function of age. Am J Psychiatry. 1975;132(9):932-7. PubMed
- Wu FC, et al. Identification of late-onset hypogonadism in middle-aged and elderly men. N Engl J Med. 2010;363(2):123-35. PubMed
Not medical advice. Written by Eric Stephens from the research cited above; not reviewed by a doctor. If something worries you, see a GP.