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TK: Professor Born, why is sleep so important for our memory?

Born: Sleep – or more precisely, deep sleep – reinforces and consolidates what we have learnt during the day. What you have to know is that everything you experience during the day is placed in a kind of interim storage depot. This is the task of the hippocampus, a part of the brain that's vital for the memory. In the evening, this means we can remember precisely and in great detail what we did during the day, where we did it and when. But if we don't consolidate these memories, they disintegrate and are forgotten. Only when they are stored in our long-term memory, which is mainly located in the cerebral cortex, do they become lasting memories. And this transfer from the hippocampus to the cerebral cortex – we call it consolidation – takes place during the deep sleep phase. I actually think the deep sleep phase is the only time when it happens.

TK: How exactly does it work?

Born: Deep sleep is when the most delta waves occur, and these are very slow. They are initiated by the cerebral cortex and cause large parts of the brain to vibrate. With these large, slow waves, the cerebral cortex tells the hippocampus: I'm ready to go, send me the information. This stimulates exactly the same neurons in the hippocampus that were stimulated when you learnt the material. This neuronal reactivation causes the memories to be transferred to the cerebral cortex, the long-term memory. What happens while we are asleep is a kind of neuronal remembering, which we call "replay". Neuronal replay is the central consolidation mechanism. And it's always necessary, no matter whether you are storing experiences, knowledge, or movements that you have learnt.

TK: What does consolidation do?

Born: One thing it does is ensure that everything we learn is placed into permanent storage. However, this changes the quality of the content. You could say it passes through a filter. Only the most important information is retained, like things that are emotionally significant, or structures and rules that you didn't even consciously recognise beforehand. 

TK: So how does the brain find these rules? 

Born: We don't yet know exactly how it happens, but even a baby can do it. Babies discover grammatical structures, for example, when they are deeply asleep. They initially make mistakes, but find a structure by always falling asleep after relatively brief phases of being awake.

TK: Is there anything I can do to boost the consolidation process? Or is there anything I should avoid?

Born: Everything that disrupts your deepest sleep also disrupts the consolidation process, like too much alcohol in the evening, or most sleeping pills. 

Learning by consolidation is particularly successful if you deliberately take another look at what you want to learn in the evening before going to bed and make it clear to yourself that you will need this knowledge later on. 

However, we can also enhance the slow delta waves that are so vital for this process. This can increase our memory performance immensely. In our experiments, we do this using a quiet sound. This is heard whenever one of these large, slow waves occurs in the brain. The sound boosts the vibration, like pushing a child on a swing. There are now a number of companies that want to use technical equipment to reproduce this effect for everyday use. However, that's all in the future. 

You can also give a person a scent to smell during their sleep that they had already smelt during the learning process. This also significantly enhances the memory. 

TK: Professor Born, thank you very much for the interview! 

About Professor Jan Born

Psychologist, memory researcher and neuroscientist Professor Dr Jan Born is the Director of the Institute for Medical Psychology and Behavioural Neurobiology at the University of Tübingen. His research into memory formation during sleep has met with international acclaim and earned him the prestigious Gottfried Wilhelm Leibniz Prize awarded by the German Research Foundation (DFG).  Jan Born is a member of the Leopoldina, the German National Academy of Sciences.