By day, the synapses, the points of contact between neurons, are reinforced by stimuli that increase their number and volume: we make new experiences, accumulate memories and skills. At night, part of this tangle breaks down: the synapses are tapered by about 20%, the unnecessary memories are eliminated and placed on those of the day to come. Confirmation that sleep is essential for “cleaning” in human memory comes from a four-year study published in Science When a brain connection is repeatedly stimulated during wakefulness, it strengthens and grows: that memory trace is consolidated, but this expansion has to be somewhat balanced to avoid saturation of possible memories. The disposal process can take place in sleep, when we pay less attention to the outside world.