Definition of night-cap:
(n) :
(countable) The time when the Sun is below the horizon when the sky is dark.
(n) :
(uncountable) Nightfall.
(n) :
(uncountable) Darkness (due to it being nighttime).
(n) :
(countable) An evening or night spent at a particular activity.
(n) :
(countable) A day, or at least a night.
(n) :
(astronomy, countable) The period of darkness beginning at the end of evening astronomical twilight when the sun is 18 degrees below the horizon, and ending at the beginning of morning astronomical twilight.