@kaia
"Your testosterone level is at its highest in the morning after you wake up. It is highest immediately after waking up from the rapid eye movement (REM) sleep stage.

The increase in this hormone alone may be enough to cause an erection, even in the absence of any physical stimulation.

As men get older, usually between ages 40 and 50, natural testosterone levels begin to fall. As this level decreases, episodes of NPT may decrease as well."