Why Can New Memories Become More Stable After a Night’s Sleep?

After learning a set of words in the evening, people sometimes retain them more securely the next day than after an equivalent period spent awake. This is not simply a matter of feeling fresher after enough rest. A central claim in sleep research is that a newly formed memory remains unstable, and sleep provides distinctive conditions for its reactivation, stabilisation and reorganisation.

During slow-wave sleep, patterns of neural activity associated with recent learning may reappear. Coordination between the hippocampus and the cerebral cortex is thought to help incorporate new memories gradually into more durable networks of knowledge. Sleep also reduces interference from the continuing arrival of new information. It is therefore less like pressing a save button and more like giving material that has already been recorded time to be sorted.

Yet “sleep makes memory better” is not a universal rule. Effects vary with the type of memory, the strength of initial learning, and the timing and quality of sleep. Direct comparisons between overnight sleep and daytime wakefulness may also contain differences in time of day, fatigue and testing conditions. Sleep can consolidate information that was encoded, but it cannot replace attention, understanding or retrieval practice. The more accurate conclusion is that memory continues to change after learning, and sleep is one important environment shaping that change.

https://pmc.ncbi.nlm.nih.gov/articles/PMC3768102/
https://www.nature.com/articles/nrn2762