What are the reasons for a car battery to discharge overnight ?

My car was running perfect until today when i was about to leave it wouldnt start and i think the battery is low in power. What could be the reasons that causes a battery to discharge overnight? It was running ok yesterday and i have not had any problem so far.|||something drawing juice from the battery like a trunk light not shutting off or a short somewhere in the system draining the battery or the battery could just be old and in need of replacement.|||could be you left the lights on, something plugged into cigarette lighter, bad battery cell, or a parasitic load ( meaning you may have a short somewhere in your wiring causing the battery to drain ) if you check and make sure everything is turned off tonight and have the same problem its probably coming from the latter to reasons. so either you'll need a new battery or you'll have to find the parasitic load|||ok first thing to check is the cables.. could be just dirty or loose.... how remove and clean the inside edges of them and retighten,,,, check the belt might be looseor old and dry...replace it older than a couple yrs..... then have the system check could be a bad battery last about three yrs or so or alternator you never know how long they will last|||sounds like your altanator is puuling the battery down, unless you left something on.|||Nothing natural could make that happen Jimmie. Have you tried exorcism?|||more info needed..what did it do when it didnt start...nothing? click? turned over but no start?|||light left on? something plugged into the cig lighter?