Sleep plays a major role in consolidating memory and managing emotions. But what happens while we sleep, by what mechanisms are information, learning, and memories stored?

The role of sleep in memory processes is fundamental. This is also the reason why we strongly advise students to sleep well before the day of an exam: not only will they be mentally and physically rested, but what they have studied will have been much better consolidated. 

This being the case, we ultimately know relatively little about these memory mechanisms, that is to say, the brain regions activated during sleep and the way in which they are organized to consolidate memory.

A Swiss team (University of Geneva) explored the subject by coupling three techniques: medical imaging (fMRI), electroencephalography (EEG), and a “neural decoder” (an artificial intelligence method). The experiments were performed on volunteers placed in an MRI. They first played two test video games (face recognition and 3D maze), and then slept for an hour or two. All the while, their brain activity was observed very closely.

Analyzing the data, the researchers found that during deep sleep, brain activation patterns were very similar to those recorded during waking play. During this phase of deep sleep, the hippocampus – one of the structures of the temporal lobe whose role is to detect new things – sends back to the cerebral cortex the information it has stored during the day. 

A dialogue then sets in and helps consolidate memory by replaying events and strengthening the link between neurons. The deep sleep phase is ideal since, at this time, the brain does not receive any external stimuli: it then takes the time and the extra resouces to evaluate all the memories to keep only the most useful by establishing this internal dialogue between the brain regions.

The researchers also dicovered something quite interesting; while the sleeping brain rethinks the two games, from the phase of deep sleep, it however focuses on the game won by the participant, and much less on the one he has lost. In the logic of evolution, it is possible that this corresponds to the need for our brain to memorize as a priority the “winning” information important for our survival, such as to avoid dangers, find food or obtain rewards.