Sleep Needed for Memory

Got kids or grandkids in school? Odds are they are not getting enough sleep, and it is hurting their learning and grades. This is a special problem for older adolescents. At this age, the biological clock shifts and makes them stay up too late if they need to get up at 6-7 A.M. to get ready for school. Kids this age need about 9 hours of sleep a night. So what is the relationship to learning? Two things:

1. When students are drowsy during class, they can't focus attention and will not encode new information effectively. Sometimes they even fall asleep in class, which means they are not encoding anything.

2. Sleep provides an uninterrupted mental environment in which the brain rehearses the events of that day. As documented in dozens of peer-reviewed research reports, this rehearsal promotes consolidation of fragile temporary memory into more permanent form.

Now, two new studies reveal what happens during sleep to accomplish this consolidation task. Just as a computer writes to a hard drive or CD for permanent storage, the brain has to have a storage mechanism. Information in the brain resides, in real time, in the form of nerve impulses flowing around in certain networks. As long as the impulses are present, the memory is present. But if the impulse patterns change, then the information they represented is lost—unless the impulse pattern was played long enough to cause structural change in the corresponding circuitry. Scientists have known for several decades that information is stored in the junctions (synapses) between neurons. We used to think that the synapses involved in learning can grow from repeated use. Impulse patterns representing the day's experiences are replayed during sleep, providing the repetition needed to stimulate growth in the corresponding synapses. But new evidence suggests that learning does not cause the involved synapses to grow, but rather prunes them during sleep to remove irrelevant information.

One of the new studies showed that synapses in mice change structure and chemistry during sleep. In sleep, the synaptic gaps become narrower and the number of neurotransmitter receptors decreases. This may constitute a pruning process. Synapses receive multiple inputs, and a pruning process could help remove irrelevant and interfering information, thus causing a relative magnification of the memory of information being rehearsed during sleep. Another way to think about it is that sleep may provide a mechanism for "smart forgetting."

The second study by another group, also in mice, confirmed this evidence of pruning and further implicated a particular receptor, the one for the excitatory neurotransmitter, glutamate. The investigators even identified the gene that is activated to remove excess glutamate receptors.

The practical application of these findings for school children is that the more they are allowed to sleep, the more time there is for sleep to cause the synaptic changes needed to store the day's learning in the "brain's hard drive." The other, more general, implication of these studies is that the brain's anatomy and physiology are readily changed by experience, a well-established fact that scientists call "neural plasticity."

Readers may be interested in "Memory Medic's" book, Memory Power 101 (Skyhorse) and his more recent book, Mental Biology (Prometheus).

Sources:

de Vivo, Luisa, et al. (2017). Ultrastructural evidence for synaptic scaling across the wake/sleep cycle. Science.  355, 507-510.

Diering, Graham H. et al. (2017). Homerla drives homeostatic scaling-down of excitatory synapses during sleep. Science. 355, 511-515.