Most healthy men often wake up in the morning with an erection. “Morning wood” and “Morning tent” are slang terms for this. Men have several erections a night as part of their sleep cycle. Many times they wake up and find they are still erect.
An erection is caused by a rush of blood to the penis. A man’s penis — especially that of a young man — can also become erect without any kind of arousing stimulation; this is known as “spontaneous erection.”
Many times it has to do with what you are dreaming about — young males can be aroused in their sleep and have a nocturnal emission (wet dream).
Morning erections are technically night time erections which happen during Rapid Eye Movement (REM) sleep (when most dreaming occurs), although men older than sixty years may have them during even non-REM sleep. Generally, males experience four intervals of REM sleep per night. These spontaneous erections are caused by specific neuro-reflexes that are stimulated during REM sleep. Morning erections are the tail end of a series of night time erections, as many as five per night that can last for 20-30 minutes each.
Many people are not aware of the fact that erections do not just serve sexual purposes; they also are a source of increased blood supply to the penis, so the male body will have several erections at night, just to keep the penis healthy.
Waking usually occurs after a session of Rapid Eye Movement (REM) sleep. Just about everyone gets an erection during REM sleep, while you’re dreaming. That’s why you often wake up from a dream, with a hard-on. Men waking up with an erection is a common occurence, but it is not something that really has to do with waking up at all.
All males, regardless of age, get erections several times during the night during REM sleep. In the beginning you cycle through the stages fairly rapidly about every 90 minutes, but as your body rests you spend less time in the more restful stages of sleep and more time in REM sleep. This means that after 7 or 8 hours of sleeping, a large part of your time is in REM sleep and thus you are likely to wake up with an erection.