While the psychological demonstrations of the effects of sleep were quite convincing, the neural mechanism by which a sleeping brain could learn, even better than while awake, remained to be identified. In 1994, neurophysiologists Matthew Wilson and Bruce McNaughton made a remarkable discovery: in the absence of any external stimulation, neurons in the hippocampus spontaneously activate during sleep. And this activity is not random: it retraces the footsteps that the animal took during the day!

As we saw in Chapter 4, the hippocampus contains place cells, i.e., neurons that fire when an animal is (or believes itself to be) at a certain point in space. The hippocampus is packed with a variety of place-coding neurons, each of which prefers a different location. If you record enough of them, you find that they span the entire space in which the animal walks. When a rat moves through a corridor, some neurons fire at the entrance, others in the middle, and yet others toward the end. Thus, the path that the rat takes is reflected by the successive firing of a whole series of place cells: movement in actual space becomes a temporal sequence in neural space.

And this is where Wilson and McNaughton’s experiments fit in. They discovered that when the rat falls asleep, the place cells in its hippocampus start firing again, in the same order. The neurons literally replay the trajectories of the preceding wake period. The only difference is speed: during sleep, neuronal discharges can be accelerated by a factor of twenty. In their sleep, rats dream of a high-speed race through their environment!

The relationship between the firing of hippocampal neurons and the position of the animal is so faithful that neuroscientists have managed to reverse the process, decoding the content of a dream from the animal’s neuronal firing patterns. During wakefulness, as the animal walks around in the real world, the systematic mapping between its location and its brain activity is recorded. These data make it possible to train a decoder, a computer program that reverses the relationship and guesses the animal’s position from the pattern of neuronal firing. When this decoder is applied to sleep data, we see that while the animal dozes, its brain traces out virtual trajectories in space.

The rat’s brain thus replays, at a high speed, the patterns of activity it experienced the day before. Every night brings back memories of the day. And such replay is not confined to the hippocampus, but extends to the cortex, where it plays a decisive role in synaptic plasticity and the consolidation of learning. Thanks to this nocturnal reactivation, even a single event of our lives, recorded only once in our episodic memory, can be replayed hundreds of times during the night (see figure 19 in the color insert). Such memory transfer may even be the main function of sleep. It is possible that the hippocampus specializes in the storage of the events of the preceding day, using a fast single-trial learning rule. During the night, the reactivation of these neuronal signals spreads them to other neural networks, mainly located in the cortex and capable of extracting as much information as possible from each episode. Indeed, in the cortex of a rat that learns to perform a new task, the more a neuron reactivates during the night, the more it increases its participation in the task during the following day. Hippocampal reactivation leads to cortical automation. Does the same phenomenon exist in humans? Yes. Brain imaging shows that during sleep, the neural circuits that we used during the preceding day get reactivated. After playing hours of Tetris, gamers were scanned the following night: they literally hallucinated a cascade of geometric shapes in their dreams, and their eyes made corresponding movements, from top to bottom.

What’s more, in a recent study, volunteers fell asleep in an MRI machine and were suddenly awakened as soon as their electroencephalogram suggested that they were dreaming. The MRI showed that many areas of their brains had spontaneously activated just before they were woken, and that the recorded activity predicted the content of their dreams. If a participant reported, for instance, the presence of people in their dream, the experimenters detected sleep-induced activity in the cortical area associated with face recognition. Other experiments showed that the extent of this reactivation predicts not only the content of the dream, but also the amount of memory consolidation after waking up. Some neurosurgeons are even beginning to record single neurons in the human brain, and they see that, as in rats, their firing patterns trace out the sequence of events experienced on the preceding day.

Sleep and learning are strongly linked. Numerous experiments show that spontaneous variations in the depth of sleep correlate with variations in performance on the next day. When we learn to use a joystick, for example, during the following night, the frequency and intensity of slow sleep waves increase in the parietal regions of the brain involved in such sensorimotor learning— and the stronger the increase, the more a person’s performance improves. Similarly, after motor learning, brain imaging shows a surge of activity in the motor cortex, hippocampus, and cerebellum, accompanied by a decrease in certain frontal, parietal, and temporal areas. Experiment after experiment gives convergent results: after sleeping, brain activity shifts around, and a portion of the knowledge acquired during the day is strengthened and transferred to more automatic and specialized circuits.

Although automation and sleep are tightly related, every scientist knows that correlation is not causation. Is the link a causal one? To verify this, we can artificially increase the depth of sleep by creating a resonance effect in the brain. During sleep, brain activity oscillates spontaneously at a slow frequency, on the order of forty to fifty cycles per minute. By giving the brain a small additional kick at just the right frequency, we can make these rhythms resonate and increase their intensity—a bit like when we push a swing at just the right moments, until it oscillates with a huge amplitude. German sleep scientist Jan Born did precisely this in two different ways: by passing tiny currents through the skull, and by simply playing a sound synchronized with the brain waves of the sleeper. Whether electrified or soothed by the sound of waves, the sleeping person’s brain was carried away by this irresistible rhythm and produced significantly more slow waves characteristic of deep sleep. In both cases, on the following day, this resonance led to a stronger consolidation of learning.

A French start-up has begun exploiting this effect: it sells headbands which supposedly facilitate sleep and increase the depth of sleep by playing quiet sounds that stimulate the slow rhythms of the nocturnal brain. Other researchers attempt to increase learning by forcing the brain to reactivate certain memories at night. Imagine learning certain facts in a classroom heavily scented with the smell of roses. Once you enter deep sleep, we spray your bedroom with the same fragrance. Experiments indicate that the information you learned is much better consolidated the next morning than if you had slept while being exposed to another smell. The perfume of roses serves as an unconscious cue that biases your brain to reactivate this particular episode of the day, thus increasing its consolidation in memory.

The same effect can be achieved with auditory cues. Imagine that you are asked to memorize the locations of fifty images, each associated with a given sound (a cat meows, a cow moos, etc.). Fifty items are a lot to remember . . . but the night is there to help. In one experiment, during the night, the researchers stimulated the subjects’ brains with half of the sounds. Hearing them unconsciously during deep sleep biased the nocturnal neuronal replay—and the next morning, the participants remembered the locations of the corresponding images much better.

In the future, will we all fiddle with our sleep in order to learn better? Many students already do this spontaneously: they review an important lesson just before falling asleep, unknowingly attempting to bias their nocturnal replay. But let’s not confuse such useful strategies with the misconception that one can acquire entirely new skills while sleeping. Some charlatans sell audio recordings that are supposed to teach you a foreign language unconsciously while you sleep. The research is clear—such tapes have no effect whatsoever. Although there might be a few exceptions, the bulk of the evidence suggests that the sleeping brain does not absorb new information: it can only replay what it has already experienced. To learn a skill as complex as a new language, the only thing that works is practice during the day, then sleep during the night to reactivate and consolidate what we acquired.

From HOW WE LEARN: Why Brains Learn Better Than Any Machine . . . for Now  by Stanislas Dehaene, published by Viking, an imprint of Penguin Publishing Group, a division of Penguin Random House, LLC. Copyright © 2020 by Stanislas Dehaene.

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