Suzana Herculano-Houzel


If you’re still one of those who think that spending a third of your life sleeping is a waste of time, take note of this: sleep is essential for consolidating the day’s memories. Without sleep, nothing happens.

From the outside, it may seem that the brain stops working during sleep. After all, the most obvious function of sleep is to “rest” the brain from the day’s activities, even if we don’t yet know what that means in cellular terms. Lose a night’s sleep, and the feeling of mental exhaustion the next day is guaranteed. Paradoxically, however, the brain doesn’t stop working during sleep; it just works differently.

One of the newly discovered consequences of this different functioning of the brain during sleep is the consolidation of memory, when the changes in brain connections that begin to be made during the day’s learning are stabilized. It’s as if the brain spends the day sketching on a notepad – the hippocampus – and at night it has the opportunity to wipe the slate clean, rewriting it in the cerebral cortex, freed, thanks to sleep, from the influences of external stimuli.

The benefits of a good night’s sleep, however, don’t apply exclusively to the tasks of the day. The simple fact of using a previously established and consolidated memory during the day makes everything start all over again: the old memory becomes vulnerable to modification and undergoes a new period of consolidation – for which sleep is fundamental.

It may seem strange to make a memory that is already well established in the brain vulnerable, but memory is nothing like videos that always show the same story every time they are played: our memories change as they are used. Thanks to this change by simply being used, the best exercise for memory is to use it – and sleep at the end of the day: as using a memory strengthens the connections involved, the more you remember something, the easier it will be to remember it in the future.

The sleeping third of life, therefore, is our ability to constantly review the events and memories of the day. Among other things, it is this ability to learn-and-consolidate-and-change-and-consolidate-again that allows us to continually improve and rewrite our life story. The brain makes its notes, writes them down, but always in pencil – which guarantees plenty of opportunities to redo it throughout life.

Originally published in Folha de São Paulo on September, 2006.

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