Why does your f-250 battery go dead overnight when nothing is left on?

Weak battery doesn't hold charge? Hidden accessory left on i.e. glove box light? Excessive parasytic load? Defective voltage regulator not opening on vehicle shut down? Try; connecting a test light in series with battery - terminal, if light illuminates there is current flow in circuit. Remove fuses one at a time until light goes out, then investigate that circuit