This paper studies the energy decoherence of an interacting quantum system. It first reviews the experiments that motivated the postulates of quantum mechanics. It then discusses a decoherence that occurs dynamically in a closed system. This effect is studied in interacting quantum systems consisting of an oscillator and spins using analytical and numerical methods. The subsequent results are contrasted with cases with no decoherence. Connections of energy decoherence with thermodynamics are explored.