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How does the brain store a memory during sleep?

What does the brain do with a memory during one night?

Cortex Thalamus Hippocampus Slow waves · cortex Spindles · thalamus Ripples · hippocampus
What comes in during the dayNight replay, from the hippocampus to the cortexLinks of the memory in the cortex

Side cross-section of the brain, simplified. The waves are drawn so they can be seen: their speeds and sizes are not to scale (a ripple is actually about 10 times faster than a spindle).

The night as text, and where these numbers come from
    With care: this page explains what research knows about sleep and memory; it does not replace medical advice. Many of these results come from small groups, from animals, or from people having surgery for epilepsy. When sleep or memory gets in the way of daily life, a doctor can look for the cause.

    Frequently asked questions about sleep and memory

    Does a nap help you remember?

    Pooled studies show no clear effect on memory alone: across 11 studies and 381 people, a short nap slightly improves thinking tests overall, but not memory clearly (Dutheil et al., 2021). In one MRI study, the more deep slow-wave sleep a nap contained, the better the images learned before it were recognized (Takashima et al., 2006).

    What happens if you do not sleep after learning?

    You remember a little less well. If you compare a person who slept after learning with one who stayed up all night, the first remembers better 58 times out of 100. An all-nighter before learning weighs more: 67 times out of 100, because the brain records less well (Newbury et al., 2021, our calculation).

    Does REM sleep help memory?

    It is not settled. The end of the night, rich in REM sleep, helps you retain a skill (Plihal and Born, 1997), and in mice the theta rhythm of REM sleep matters (Boyce et al., 2016). But in humans, reducing REM sleep with an antidepressant does not harm memory (Rasch et al., 2009).

    Can a memory be strengthened during sleep?

    Yes, a little, in the lab: a sound heard while learning, replayed during slow-wave sleep, slightly improves the memory of what went with it: the person who hears the sound again remembers better 58 times out of 100. During REM sleep or while awake, there is no effect (Hu et al., 2020). It reactivates a memory that was already learned.

    Does the brain really replay the day?

    In rats, yes: hippocampal neurons that were active together during exploration light up together again in the slow-wave sleep that follows, sped up (Wilson and McNaughton, 1994; Rasch and Born, 2013). In humans, it is seen indirectly, through recorded waves and imaging.

    See also

    How do neurons communicate with each other? What happens in the body during sleep? What does an all-nighter do to your brain?

    Sources

    1. Rasch B, Born J. About sleep's role in memory. Physiological Reviews, 2013. doi:10.1152/physrev.00032.2012
    2. Buzsáki G. Hippocampal sharp wave-ripple: a cognitive biomarker for episodic memory and planning. Hippocampus, 2015. doi:10.1002/hipo.22488
    3. Wilson MA, McNaughton BL. Reactivation of hippocampal ensemble memories during sleep. Science, 1994. doi:10.1126/science.8036517
    4. Girardeau G et al. Selective suppression of hippocampal ripples impairs spatial memory. Nature Neuroscience, 2009. doi:10.1038/nn.2384
    5. Staresina BP et al. Hierarchical nesting of slow oscillations, spindles and ripples in the human hippocampus during sleep. Nature Neuroscience, 2015. doi:10.1038/nn.4119
    6. Ngo HVV et al. Auditory closed-loop stimulation of the sleep slow oscillation enhances memory. Neuron, 2013. doi:10.1016/j.neuron.2013.03.006
    7. Gais S et al. Learning-dependent increases in sleep spindle density. Journal of Neuroscience, 2002. doi:10.1523/JNEUROSCI.22-15-06830.2002
    8. McClelland JL, McNaughton BL, O'Reilly RC. Why there are complementary learning systems in the hippocampus and neocortex. Psychological Review, 1995. doi:10.1037/0033-295X.102.3.419
    9. Takashima A et al. Declarative memory consolidation in humans: a prospective functional magnetic resonance imaging study. PNAS, 2006. doi:10.1073/pnas.0507774103
    10. Kim JJ, Fanselow MS. Modality-specific retrograde amnesia of fear. Science, 1992. doi:10.1126/science.1585183
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    12. Plihal W, Born J. Effects of early and late nocturnal sleep on declarative and procedural memory. Journal of Cognitive Neuroscience, 1997. doi:10.1162/jocn.1997.9.4.534
    13. Boyce R et al. Causal evidence for the role of REM sleep theta rhythm in contextual memory consolidation. Science, 2016. doi:10.1126/science.aad5252
    14. Rasch B et al. Pharmacological REM sleep suppression paradoxically improves rather than impairs skill memory. Nature Neuroscience, 2009. doi:10.1038/nn.2206
    15. Newbury CR, Crowley R, Rastle K, Tamminen J. Sleep deprivation and memory: meta-analytic reviews of studies on sleep deprivation before and after learning. Psychological Bulletin, 2021. doi:10.1037/bul0000348
    16. Hu X, Cheng LY, Chiu MH, Paller KA. Promoting memory consolidation during sleep: a meta-analysis of targeted memory reactivation. Psychological Bulletin, 2020. doi:10.1037/bul0000223
    17. McGraw KO, Wong SP. A common language effect size statistic. Psychological Bulletin, 1992. doi:10.1037/0033-2909.111.2.361
    18. Dutheil F et al. Effects of a short daytime nap on the cognitive performance: a systematic review and meta-analysis. International Journal of Environmental Research and Public Health, 2021. doi:10.3390/ijerph181910212

    True or false?

    A memory stays in the hippocampus for life.

    False. Over time, recall relies less on the hippocampus and more on the cortex (Takashima et al., 2006). In rats, damaging the hippocampus 7 to 28 days after learning no longer erases the memory (Kim and Fanselow, 1992). Another theory holds that very detailed memories always depend on it (Squire et al., 2015).

    Without REM sleep, you retain nothing.

    False. Men whose REM sleep was reduced by an antidepressant retained words and skills just as well (Rasch et al., 2009). Its exact role is still debated.

    An all-nighter before learning hurts memory more than an all-nighter after.

    True. In the pooled studies, the person who slept remembers better 67 times out of 100 when the all-nighter comes before learning, compared with 58 times out of 100 when it comes after (Newbury et al., 2021, our calculation).

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    Metabolive. "How does sleep store memories?" Updated November 2, 2026. https://metabolive.com/en/brain/sleep-and-memory.