The mysteries of sleep are beginning to unravel thanks to a new study that reveals how the brain transforms the day’s events into fully encoded memories while we slumber. Appearing in the journal Science, the new research indicates that the fine-tuning of these memories occurs during periods of extremely low brainwave activity, when the vast majority of cortical neurons switch off in order to allow only those involved in creating a specific memory to do their work.

Scientists have known for decades that during sleep, neurons across the cortex oscillate between bursts of activity and periods of silence. Because the active phase is immediately preceded by a surge of activity in the hippocampus – the part of the brain that processes memory – it has long been assumed that this oscillation somehow serves to encode recent events into fixed memories.

During the silent phase of the cycle, neurons across the cortex synchronize to the slowest type of brainwave, known as delta waves, leading scientists to believe that the neurons are simply resting during this period. However, the new study reveals for the first time that not all of the cortical neurons actually go to sleep, and that a very small number actually remain active and group together to form neural connections that create specific memories.

The study authors trained rats to perform a spatial memory task, and recorded their hippocampal and cortical activity both during the task and during the first two hours of sleep. They discovered that while the rodents were snoozing, their hippocampi spontaneously repeated the activity patterns it had displayed during the memory task, indicating that it was replaying the memory of the event.

As expected, this caused a spike of activity across the cortex, followed by delta waves and the corresponding neuronal silence. Yet each time this happened, a small minority of cortical neurons remained active while the rest slept. By observing this process, the team discovered that the activity patterns within the hippocampus actually predicted which cortical neurons would remain active, allowing them to watch in real-time as short-term memories in the hippocampus were recorded as stable memories in the cortex.

To confirm their finding, the researchers later artificially stimulated the cortices of the sleeping rats, causing the wrong neurons to remain active during the delta phase, and found that this prevented the animals from being able to complete the spatial memory task the next day.

Above all, this discovery is significant as it radically alters our understanding of delta waves and their purpose: rather than simply sending the brain to sleep, these waves exist in order to isolate the specific neurons that are needed to form a memory, suppressing all the others so that those selected can get to work.