Why does "morning stand" happen?

As for men, it is physiological phenomenon "morning standing" thought that everyone has experienced once, why does "morning stand" occur?

Apparently the mechanism that causes it seems to be in the autonomic nervous system.

Details are as below.
FAQ: Why do guys get "morning wood?"

According to this page, the erection is controlled by the autonomic nervous system, and it seems to be caused by a parasympathetic nerve functioning when relaxed. On the contrary when the lion is chased and it is clogged down, sympathetic nerve works so that erection does not occur. And in the morning human beings are in the most relaxing state, so it is said that morning starts.

Morning standing (physiological phenomenon) - Wikipedia

According to the above page, sleep has REM sleep and non-REM sleep which alternate periodically, and it is said that it is said that dreams are easy to see, a certain nerve is stimulated during REM sleep, and it seems to erect while sleeping . And, in addition to visiting REM sleep for about 90 minutes cycle in about 90 minutes cycle, people often wake up because there are many cases of REM sleep, which is shallow asleep, so if it is maintained until morning it will be a morning stick.

Incidentally, the morning stand is applied to the differential diagnosis of erectile dysfunction (ED) and it seems that you can check whether erectile dysfunction is psychogenic using erection or sleeping erection.