Why do guys get morning wood

It's not so much to do with the morning as the night. You get erections several times during the night when you sleep. While you are in your REM stage you almost always get an erection.

This following is from : http://www.mybrotha.com/mens-sexual-health1.asp

The erections you refer to are actually night time (nocturnal) erections which happen while you're asleep. Men begin having them in early childhood and they continue throughout adult life. They appear during REM (Rapid Eye Movement) sleep -- the stage of sleep where dreaming occurs.

However, these involuntary erections have nothing to do with dreams or sexual excitement. They occur when neurological stimulators cause blood to flow into the penis. This ensures that oxygen is properly delivered to the penis and helps keep penile tissue healthy.

Men don't normally notice these erections, but may occasionally encounter them upon waking. About 4 or 5 of these hard episodes happen every night, and yes -- all men have them. Men who are unable to have an erection because of a psychological problem still have erections during the night.

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It's also a blood pressure issue. When sleeping, your blood pressure drops pretty significantly. The first morning surge of pressure as you wake up can cause morning wood like standing in front of the class as a teenager.

Nice dreams at night.

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Gentlemen's testosterone levels are highest in the morning. Their blood pressures also rise as they awake (as previously answered) and bring a surge. Also thinking about the fine women they just met can inspire nocturnal excitement. Since dreams happen largely in the twenty minutes before waking up, these thoughts rise in the sub-conscious early in the morning. - Dr. Francis Papageorgio (PH.D. - University of Central North Dakota)