Some mobs in the Treasures of Aht Urhgan areas are nocturnal: they sleep during the daytime, and are active during the nighttime (but the specific hours may vary 
). When they are asleep, they do not move around, aggro or link, even though they might normally do so.
Nocturnal is the opposite of Diurnal.
Jnun are known to be nocturnal. Eruca, Ziz, and Wild Karakul are known to be diurnal.