Sunlight exposure is a hidden motive possessed by vampires in The Sims 3: Late Night. It decreases every time a vampire is exposed to sunlight. (Players familiar with PlantSim motives from The Sims 2: Seasons will recognize this as the inverse of PlantSims' Sunlight need.)
This motive can be noticed when a vampire goes into shade for a few minutes after getting the "Heating Up" moodlet, then goes back into sunlight and regains the moodlet shortly afterward. This example shows that the need to keep a vampire Sim in the shade must last longer than the moodlet.