Thanks for query,
Nothing to worry,this is quite normal.
Now a days to see exciting things in daily routine life,seeing porno or sex movies will unconsciously affect the mind and during night time resulting into night discharge.
Do not worry about sympathetic or para sympathetic nerve
Just relax,wear light and loose clothes at night.
See some light movie or TV serial or some reading before going to bed.
Pray the god and go for sleep.
Ok and bye.