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Sleep and memory

In 1924 two Cornell researchers had two colleagues learn nonsense syllables and then either sleep or stay awake. After sleep they forgot less.

This course gives general information about how memory research works. It is not medical advice. If memory problems worry you, talk to a doctor.

Students often trade sleep for study time the night before an exam. Research on sleep and memory suggests that trade is a poor one, though the details are still argued over. This unit covers what happens during a night of sleep, the first experiment on sleep and forgetting, the two main explanations, and where scientists disagree.

A night in stages

Sleep is not one steady state. OpenStax describes three stages of non-REM sleep and one stage of REM sleep, told apart by brain waves. Stage 2 contains sleep spindles, short bursts of faster waves that may matter for learning and memory. Stage 3 is deep or slow-wave sleep, with slow, large delta waves under 3 Hz. REM sleep, named for rapid eye movements, has busy, wake-like brain activity while the body's muscles go slack, and it is when most vivid dreams happen. Through the night the sleeper cycles through these stages several times, with more deep sleep early in the night and longer REM periods toward morning.

A simplified hypnogram for about eight hours: W awake, R REM sleep, N1 to N3 non-REM stages. Deep N3 sleep clusters early; REM periods lengthen toward morning. The shape is a teaching sketch, not one person's recording.

The first experiment

BackgroundEbbinghaus had noticed that his 24-hour savings were nearly as high as his 9-hour savings, a smaller drop than the rest of his curve would predict, and the 24-hour interval included a night of sleep. John Jenkins and Karl Dallenbach at Cornell followed up in 1924. Two colleagues learned lists of nonsense syllables either in the morning or late in the evening, and were tested after intervals between one and eight hours, spent either awake or asleep. Forgetting curves were much flatter when the interval was filled with sleep. Jenkins and Dallenbach concluded that forgetting is less a matter of old impressions decaying than of interference from new experiences.

Two explanations, and the argument

The first explanation is passive: sleep simply shields new memories because nothing new comes in to interfere. The second is active: during sleep the brain replays and strengthens recent memories and links them with older knowledge, a process called consolidation. A 2013 review by Björn Rasch and Jan Born, with 1,358 references, gathered human and animal evidence that sleep does something for memory, and many studies tie deep sleep and spindles to fact learning. Yet the size and mechanism of the effect remain in dispute. A 2021 study in the Journal of Sleep Research, titled "Evidence against a large effect of sleep in protecting verbal memories from interference", found no large protective effect in its tests. A 2016 reanalysis of two earlier data sets found that sleep did both: it protected memories, and it gave an even larger boost to memories that had been too weak to recall before sleep. The early studies also mixed sleep with time of day and tiredness, a problem modern designs try to remove.

What can a learner take from this without overreaching? Studying and then sleeping is a reasonable plan, and giving up a night of sleep to cram trades the consolidation time and next-day alertness for a few extra hours with material that will mostly be forgotten anyway. That is general information; if you have ongoing sleep problems, a doctor is the right person to ask.

1885: Ebbinghaus publishes his forgetting curve; the 24-hour point is higher than expected.
1924: Jenkins and Dallenbach: less forgetting after sleep than after waking.
1953: Aserinsky and Kleitman report REM sleep.
2013: Rasch and Born review sleep and memory with 1,358 references.
2015: Murre and Dros replicate Ebbinghaus and see a jump at 24 hours.
2021: A study reports evidence against a large protective effect of sleep on verbal memory.

Sleep spindle	Burst of faster waves in stage 2; may matter for memory
Slow-wave sleep	Stage 3, delta waves under 3 Hz, mostly early in the night
REM sleep	Wake-like brain activity, slack muscles, vivid dreams
Consolidation	Turning new learning into lasting long-term memory
Jenkins and Dallenbach (1924)	Two colleagues; less forgetting over 1 to 8 hours of sleep

Key points

Sleep cycles through non-REM stages 1 to 3 and REM several times a night.
Jenkins and Dallenbach (1924) found less forgetting after sleep.
Explanations: protection from interference, active consolidation, easier access.
The size of the effect is still debated; early studies had confounds.

Which stage has slow delta waves under 3 Hz?
- Stage 1
- Stage 2
- Stage 3
- REM
Sleep spindles appear mainly in which stage?
- Stage 1
- Stage 2
- Stage 3
- REM
In REM sleep, what are the muscles like?
- Tense
- Slack
- Twitching constantly
- Same as when awake
How many people learned the syllables in Jenkins and Dallenbach (1924)?
- Two
- Twenty
- Two hundred
- One thousand
What did Jenkins and Dallenbach think caused most forgetting?
- Decay alone
- Interference from new experiences
- Too much sleep
- Poor eyesight
Which explanation says the brain replays and strengthens memories during sleep?
- Passive protection
- Active consolidation
- Sensory memory
- Proactive interference
Scientists fully agree on how large the sleep effect on word memory is.
- Yes
- No
What was a weakness of many early sleep studies?
- They used too many people
- Sleep was mixed up with time of day and fatigue
- They used modern brain scans
- They studied only REM
When is deep slow-wave sleep most common?
- Early in the night
- Just before waking
- Evenly spread
- Only in naps

Sources for this course (16)

OpenStax Psychology 2e (openstax.org/books/psychology-2e)
sections 8.1 How Memory Functions (Atkinson and Shiffrin 1968; Baddeley and Hitch 1974; Miller 1956; Cowan 2010; Peterson and Peterson 1959; Keppel and Underwood 1962; Tulving 1972)
8.2 Parts of the Brain Involved with Memory (H. M.)
8.3 Problems with Memory (Ebbinghaus 1885; Schacter 2001 seven sins; Loftus and Palmer 1974; proactive and retroactive interference)
8.4 Ways to Enhance Memory, 4.3 Stages of Sleep. Sperling (1960) Psychological Monographs 74(11) (sites.socsci.uci.edu/~whipl/staff/sperling/PDFs/Sperling_PsychMonogr_1960.pdf): 12 symbols, 76% partial report, about 9.1 letters, whole report slightly over four. Ebbinghaus (1885/1913) savings 58.2% at 20 min, 44.2% at 1 h, 35.8% at 8.8 h, 33.7% at 1 day, 27.8% at 2 days, 25.4% at 6 days, 21.1% at 31 days
Murre and Dros (2015) PLoS ONE 10(7) e0120644 replication (one subject, 70 hours, rows of 13 syllables)
Cepeda et al. (2006) Psychological Bulletin, PubMed 16719566 (839 assessments, 317 experiments, 184 articles)
Cepeda et al. (2008) Psychological Science 19(11)
PubMed 19076480 (more than 1,350 people, gap up to 3.5 months, test up to 1 year, optimal gap about 20 to 40% of a 1-week delay and 5 to 10% of a 1-year delay)
Roediger and Karpicke (2006) Psychological Science 17(3) 249-255 (Experiment 2: 5-min test SSSS 83%, SSST 78%, STTT 71%; 1-week test STTT 61%, SSST 56%, SSSS 40%)
Dunlosky et al. (2013) PSPI 14(1)
PubMed 26173288. Jenkins and Dallenbach (1924) American Journal of Psychology 35, 605-612 (two colleagues, nonsense syllables, 1 to 8 hours)
Sara (2017) J Neurosci review, PMC6596760 (Rasch and Born 2013, 1,358 references)
Journal of Sleep Research (2021) 10.1111/jsr.13042 "Evidence against a large effect of sleep in protecting verbal memories from interference". Roediger and McDermott (1995) JEP:LMC 21 (false recall 40% with 12-word lists, 55% with 15-word lists)
Card intervals in the hands-on parts come from this site's own flashcard code (boxes 0 to 5, 0, 1, 3, 7, 14, 30 days)
Everyday scenes are made up for teaching. General information only, not medical advice.

Checked: 2026-10-17