This is the 1117th most frequent Dutch word.
nachts
"At night."
's nachts is used here to indicate the nighttime as a general period during which the studying occurs. It functions as an adverbial phrase pertaining to time.
Ik studeer vaak 's nachts wanneer het stil is.
I often study at night when it is quiet.
's nachts is employed here to denote the time period during which the stars are visible.
De sterren zijn 's nachts goed te zien.
The stars can be seen well at night.
's nachts indicates the period of the night in which the action of walking takes place.