(CNN) — Scientists have long known our brains need sleep to review the day’s events and transfer them into longer-term memories. Students are often told to study just before turning in to maximize their recall of material for a test the next day.

But the exact way in which the brain stores our memories is poorly understood.

Now for the first time, tiny microelectrodes planted inside the brains of two people show just how the brain’s neurons fire during sleep to “replay” our short-term memories in order to move them into more permanent storage. The study was published Tuesday in the journal Cell Reports.