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Physical activity, education and intellectually demanding jobs been found to boost this reserve. These are protective factors that improve the brain’s resilience to damage. But there’s another X factor, which scientists have struggled to identify. A new study suggests it may be sleep, and in particular deep sleep.

That’s the stage when the body repairs bones, muscles and the immune system. It’s also when the brain consolidates new memories. If you burn the candle at night, you’ll be more susceptible to illness and more likely to forget things you recently learned.