Built up urine in your penis from the night of sleeping
and
You have more testosterone during the mornings (not true for every man though)
??
Testosterone is highest in most people around 7 to 8 AM, lowest twelve hours later. (Although, oddly, strength seems to be highest at around 3 PM)
Which is like a modern age joke on some men, .... come home from the labours of the day (stress) - at your lowest point of testosterone (8 PM) - and then jump the SO with quality wood? (!)
7 AM is also supposed to have the highest number of heart attacks for the day. (Heart rate? Hydromaxmmm...)
Anyway, ...
It's not from built up urine.
First off, it happens several times over the course of sleep. If one wakes up during one of the Up cycles, -- (you notice that) you've got wood. Unless there's physical damage it happens repeatedly all the time.
Up is the (healthy) penis' natural state. There's a constrictor valve to -keep- blood out of there. rofl
(and then there's a secondary-stage response involving production of NO directly from the endothelial cells)
Spontaneous nocturnal erections, (NPT - nocturnal penile tumescence) seems to be triggered by dreams. There have been several studies 'confirming' this since the 1960s, but some of the more recent studies have been performed to look at the difference in response to the different cycles of REM sleep (dreaming).
That is ... there's a correlation between dreaming and night wood, but it's a loose one that can proceed or lag the reading of the REM activity. The offset seems to becomes less the longer one has been asleep.
Other studies on this ^ have found a tighter correlation to heart rate -during- the REM sleep. A calm dream (or calm segment of a longer dream) may not cause night wood, but if the dream increases heart rate then apparently up goes the johnson.
I find that unlikely for all cases, ... if the heart rate was increased due to a night terror it seems unlikely the body's response to that would be arousal. They could do another study on victims of chronic nightmares and see if their heart rate correlation still ... stands up.
Which is not to say they're wrong, ... just that apparently most dreams .. increasingly across the sleep period..., after your brain works out all the stress over what an ass your boss is, etc, the dreams turn more and more to sexual content, higher heart rate (not tied to 'physical' danger like being chased by wolves or whatever) , arousal , wood.
I have become very worried as I read in one post of everyday morning erections. I dont have them and havent had them in more than a year,
You might just be waking up during a down cycle.
The cheapy test for proper physical function is supposed to be wrapping several postal stamps around your penis so they make a band that: won't fall off accidentally, -- but also is so small that they will break under the expansion of an erection -- indicating proper physical function during the previous sleep.
yet when I masturbate I get very good erections,
You're fine.
.