There's already a strong link between sleep and memory, and scientists have just found out more about how that relationship works: there are specific patterns of brain activity that open up windows on our past experiences, fixing them in our long-term memory.

These patterns involve the slow oscillations (SOs) of brain waves that normally accompany sleep, and the sharper sleep spindle bursts of activity that happen during dreamless slumber. It now seems that the precise way these two types of brain activity coordinate with each other makes a big difference in how well we remember something.

Our memories are essentially being reactivated during sleep via these two brain activity patterns, the researchers suggest, making us more likely to remember them. The stronger the reactivation, the more likely we are to be able to recall a memory later on.

"Our main means of strengthening memories while we sleep is the reactivation of previously learnt information, which allows us to solidify memories in neocortical long-term stores," says neuropsychologist Bernhard Staresina from the University of Birmingham in the UK.

"We have discovered an intricate interplay of brain activity – slow oscillations and sleep spindles – which create windows of opportunity enabling this reactivation."

Across two experiments, 20 volunteers were asked to remember links between words and scenes, and words and objects, before taking a nap for at least half an hour. Thoughts about scenes and objects trigger different parts of the brain, so using them enabled the researchers to identify which memories were being reactivated.

Using electroencephalogram (EEG) scans while the participants were sleeping, and then testing them on what they'd remembered when they woke up, the team was able to make the link between the tight SO-spindle coupling and improved recall.

Both SOs and sleep spindles are required to reactivate memories, the researchers observed, and the closer the SO-spindle sync, the stronger the reactivation – and the better the preservation of the memory.

This is what the researchers expected, based on several previous studies, but the relationship between the two types of brain activity and cues involving memory hasn't been measured in this much detail before – it fills in some of the gaps in our understanding about how the brain consolidates memories while we're sleeping.

It's yet more evidence, as if we needed it, of the benefits of sleep. Besides making sure our memories stay in place, getting the right amount of shut-eye can improve mental agility and sort through the events of the day.

Further research is going to be required to look more closely into the relationship between SOs, sleep spindles, and memory. These experiments only involved two-hour naps rather than full nights of sleep, and the researchers say they want to take a closer look at related interactions with other parts of the brain, like the hippocampus.

"These results shed new light on the memory function of sleep in humans and emphasize the importance of orchestrated sleep rhythms in strengthening our powers of recall and orchestrating the creation of memories," says neuropsychologist Thomas Schreiner from the Ludwig Maximilian University of Munich in Germany.

The research has been published in Nature Communications.