Memory and forgetting are everyday phenomena, and though they have been studied for over a century the processes which underlie them have not been easily understood. Early in the twentieth century it was thought that forgetting was due to decay of memory traces over time. This account of forgetting suggested that decay was perhaps a natural consequence of the passage of time. But a competing theory of memory postulated interference as the culprit of forgetting: either prior learning (proactive interference) or later learning (retroactive interference) were what caused us to forget (Wixted, 2004). So which explanation best explained forgetting, interference or decay? Jenkins & Dallenbach (1924) found that subjects remembered more nonsense syllables after a sleep-filled delay than after an equal delay period in which subjects remained awake. Decay theory would predict no difference between the conditions, since it postulated that the passage of time was responsible for forgetting. But interference theory predicted less forgetting after sleep due to the absence of new learning. Hence decay theory was abandoned as the sole explanation of forgetting.