Previous research has shown that sleep can selectively enhance memories needed for the future
, and sleep deprivation can even lead to false memories
. However, a new study
published in the journal Cortex says not only does sleep protect memories, it also makes them more easily accessible.
Experimental psychologist Nicolas Dumay, of the University of Exeter and the Basque Center on Cognition Brain and Language, re-analyzed two previously published studies on the connection between declarative memory (remembering facts)
and sleep. In both studies, participants took memory tests immediately after being trained on made-up words such as "frenzilk" and "caravot." They then took a retest after a 12-hour period of sleep or wakefulness.
Dumay reviewed this previous research to identify which words were remembered at initial testing and then remembered at retesting versus which words which were not initially recalled but later "gained" after the 12-hour period.
He studied 72 of the original participants, excluding those whose word recall was close to 0% or 100% at the initial test. Eliminating these outliers and analyzing the middle group provided an even assessment to examine both the beneficial effect of sleep and the detrimental effect of active wakefulness, Dumay said.
In comparison to daytime wakefulness, a night's sleep helped participants access unrecalled memories more than it prevented forgetting, according to the study.
"Sleep is good not just to maintain secured knowledge, but also to rescue weaker information, which we could not remember while still awake," Dumay told CNN. "The light here is on those inaccessible memories, which sleep makes suddenly more accessible."
In fact, Dumay said sleep nearly doubles our chances of remembering previously unrecalled material.
Though the data only covers word recall, Dumay said that the findings could translate to most declarative memories, including "facts about the world (semantics) and about our own experience (life events)."
The study attributes this "post-sleep boost in memory accessibility" to the hippocampus, a part of the temporal lobe of the brain.
"The hippocampus is thought to be responsible for new memories," said Dr. Clete Kushida, a professor of psychiatry and behavioral sciences at Stanford School of Medicine and director of the Stanford Sleep Medicine Center. "It is suggested that sleep is associated with hippocampal encoding and consolidation of memory."
Kushida, who is not affiliated with the study, found its findings to be interesting. "The data presented in the article provides convincing evidence that sleep promotes access to memory traces that may have been initially too weak to be retrieved," said Kushida, but he emphasized further evidence is needed to bolster this hypothesis.
Though Dumay said revisiting data collection has proven successful, and should encourage researchers to do so more often, more research is indeed needed to understand whether these gains are associated with other changes in memory.