Night or night time is the period after all sides have ended their turn for the day, at which point the sun sets. Night ends at dawn, and sides can once again start taking turns.
Units sleep at night, and not sleeping or not sleeping enough can impair their performance the next day.
Unit's Move drops to zero at the end of turn, even if they had some left over, so it's always zero at Night. Rules of acting off-turn are in place, so spells costing juice cannot be cast.Erf-b2-p33a
Currently, it is unknown how long a night lasts, or whether it is fixed or not.