Does Sleep Help Memory? What Happens to What You Learned While You Sleep
Hiroki Obara · Developer
From April 14 to June 7, 1923, two seniors at Cornell University lived in the psychology laboratory. Between 11:30 p.m. and 1 a.m., they learned a list of ten nonsense syllables and went straight to bed. One, two, four or eight hours later, they were woken and asked to recall the syllables.
The next year, John G. Jenkins and Karl M. Dallenbach published the results in The American Journal of Psychology. After sleep, the two recalled more than twice as many syllables as after the same time spent awake.
This post starts from that experiment and covers sleep after learning, sleep before learning, and what seems to happen in the sleeping brain, kept to what the papers and their abstracts report.
The 1924 experiment
Each list had ten syllables, and the students repeated it until they could recite it correctly once. For the daytime sessions, most learning took place between 8 and 10 a.m. The students then went about their day and came back to the laboratory at a set hour to recall the list.
On average, the two recalled 5.8 and 6.0 syllables after sleep, and 2.4 and 2.8 syllables after staying awake. The gap was there even after one hour: 7.1 and 7.0 syllables after sleep, 4.4 and 4.8 after waking.
The shape of the results over time was different too. When the students stayed awake, they recalled fewer syllables the longer they waited. After sleep, recall dropped a little at first, then stayed at about the same level from two hours on.
The paper is open about the study’s weak points. After a few nights, the students became hard to wake. They would get out of bed, go to the next room, give their answers, and say the next morning that they remembered nothing of it. The experimenter judged from their behavior that they were awake. And there were only two of them.
Is forgetting a matter of time?
At the end of the paper, Jenkins and Dallenbach conclude that forgetting is not so much the decay of old impressions as the interference, inhibition or obliteration of the old by the new. A footnote adds that the advantage of studying at night becomes evident.
On this view, sleep protects memory by being a time when nothing new comes in.
In 2006, J. M. Ellenbogen and colleagues tested this idea in a study published in Current Biology. They expected that the benefit of sleep for declarative memory, the hippocampus-dependent kind, would show up when that memory was challenged with competing information. According to the abstract, sleep protected declarative memories from later associative interference. The authors describe it as the first study to show this.
Sleeping within a few hours of learning
Also in 2006, S. Gais and colleagues in Germany published two experiments in Learning & Memory on how well high school students recalled vocabulary.
According to the abstract, memory for the words improved when sleep came within a few hours of learning. This held regardless of the time of day, and under conditions where interference while awake was kept equal. Going without sleep made memory worse, and the difference was still there after a night of recovery sleep. The authors conclude that tiredness building up during waking cannot explain the results.
Skills change overnight too
Memory is not only about words. A 2002 study in Neuron by M. P. Walker and colleagues looked at motor skill practice, with and without sleep afterwards.
According to the abstract, a night of sleep increased the speed of the movement by 20 percent, with no loss of accuracy. The same amount of time spent awake gave no significant improvement. The overnight gain was related to the amount of stage 2 non-REM sleep, especially late in the night.
What happens in the sleeping brain
In Jenkins and Dallenbach’s day, sleep was seen as a passive state that protected memory by keeping interference away.
In 2013, B. Rasch and J. Born wrote a review in Physiological Reviews of more than a century of research on sleep and memory. According to its abstract, current theories see sleep as an active period in which memories go through “system consolidation”.
The abstract describes the process this way. During slow-wave sleep, newly learned memories are reactivated in the brain, then transformed and built into long-term memory. The REM sleep that follows may stabilize these transformed memories. Earlier research focused on REM sleep, but newer work shows that slow-wave sleep plays a central part in consolidation.
The review puts it as a division of work: the sleeping brain is in a state suited to consolidating memories, and the waking brain to taking in new ones.
Sleep before learning
So far, all of this is about sleep after learning. A 2007 study in Nature Neuroscience by S. S. Yoo and colleagues looked at sleep before learning.
According to the abstract, after one night without sleep, people showed less activity in the hippocampus while taking in new events, and they remembered less of them later. The authors write that sleep before learning seems to matter for preparing the brain to form new memories the next day.
What happened to attention when people slept less for two weeks is covered in Is Six Hours of Sleep Enough? Experiments on a short nap after learning, and on learning at night and reviewing in the morning, are in Ten Study Methods That Help You Remember.
What these studies show, and what they don’t
The studies here share one result. People who slept after learning remembered more than people who stayed awake, for words and for a motor skill alike. When people went without sleep before learning, their ability to take in new memories was weaker.
On the other hand, the 1924 study had only two participants, and it is not clear that they were fully awake when they gave their answers at night. The review’s account of memories being reactivated in the sleeping brain is also described there as coming mainly from research on hippocampus-dependent memory.
References
- Jenkins JG, Dallenbach KM. Obliviscence during sleep and waking. Am J Psychol. 1924;35(4):605-612.
- Walker MP, Brakefield T, Morgan A, Hobson JA, Stickgold R. Practice with sleep makes perfect: sleep-dependent motor skill learning. Neuron. 2002;35(1):205-211.
- Gais S, Lucas B, Born J. Sleep after learning aids memory recall. Learn Mem. 2006;13(3):259-262.
- Ellenbogen JM, Hulbert JC, Stickgold R, Dinges DF, Thompson-Schill SL. Interfering with theories of sleep and memory: sleep, declarative memory, and associative interference. Curr Biol. 2006;16(13):1290-1294.
- Yoo SS, Hu PT, Gujar N, Jolesz FA, Walker MP. A deficit in the ability to form new human memories without sleep. Nat Neurosci. 2007;10(3):385-392.
- Rasch B, Born J. About sleep’s role in memory. Physiol Rev. 2013;93(2):681-766.
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