Sleep and forgetting
Why do we sleep? Even after decades of investigation, this simple question remains an open issue. Indeed, there is no single answer, and complementary functional hypotheses have been suggested. For instance, it has been proposed that we sleep in order to preserve energy (Berger & Phillips, 1995), to keep cerebral thermoregulation constant (McGinty & Szymusiak, 1990), to detoxify neural cells (Inoue, Honda, & Komoda, 1995), to restore tissues (Adam & Oswald, 1977), and to preserve genetically programmed behavioural patterns (Jouvet, 1991). An additional hypothesis of interest is that sleep aids the long-term storage of memories recently acquired during wakefulness, and thus that it helps to prevent forgetting. Quintilien raised a similar idea in the 1st century AD (see Dudai, 2004). However, it was not until the beginning of the 20th century that this hypothesis was tested empirically. The ﬁrst known experimental study on this matter was performed by Jenkins and Dallenbach in 1924. They showed that the classical Ebbinghaus forgetting curve for nonsense syllables was markedly dampened if the time between learning and recall was spent asleep, as opposed to time spent in the waking state. However, according to these authors and their immediate successors (e.g., Newman, 1939; Van Ormer, 1933), sleep merely had a passive role in the prevention of oblivion, by protecting novel memories from the intrusion of interfering information arising during wakefulness.