People think that the purpose of sleep is to rest the brain. But there is clear evidence that the brain is still busily at work during sleep, even when the brain is not dreaming.
No, I am not talking about “sleep learning,” where the idea is that you play recordings of information while you sleep. There is not much evidence that this works.
Did you know that your brain works while you sleep? Yes, both during dreaming and non-dreaming, your brain is consolidating memories of events in the immediately preceding day.
Most people think that the purpose of sleep is to rest the brain. But there is clear evidence that the brain is still busily at work during sleep, even when the brain is not dreaming. Decades ago, researchers demonstrated that many neurons fired just as much during sleep as during wakefulness. Some neurons were even more active during sleep.
One advantage that sleep provides for memory consolidation is that the brain doesn’t have all the distractions that occur during daytime wakefulness. Multiple-conflicting stimuli and tasks are very disruptive to memory consolidation.
The advantages offered by having fewer disruptive influences during sleep have also been confirmed in a study conducted in the brain imaging lab of Thomas Pollmacher in Munich, Germany. An auditory text stimulus was presented to sleep-deprived subjects prior to and after the onset of sleep, and imaging was performed to compare wakeful responses to sound stimuli with those during various stages of non-dreaming sleep. Brain activity during sleep was suppressed in auditory pathways and visual cortex, including other brain regions that are interconnected with the visual cortex. Suppression suggests that sleep shields the brain from the arousing effects of external stimulation that might disturb sleep. Blocking out such interference effects should facilitate memory consolidation. This study also prompted researchers to conclude that consolidation of memory occurs over many hours, at least in sleep-deprived subjects. That is to be expected, inasmuch as consolidation of memory depends on protein synthesis and physical changes in synapses.
Students often cut back on sleep to finish ever-mounting piles of homework and study. Combat soldiers are trained to function under sleep-deprived conditions. But these strategies are likely counter-productive. At my university, our Corps of Cadets used to have a tradition of rousing freshmen in the middle of the night and preventing them from sleeping. The idea was to make them tough. More likely, it just made them unable to do well in school, as I have seen many of them flunk out. Another area where this problem has surfaced is with sleep-deprived medical residents.
Sleep loss degrades many brain functions. In one study, sleep loss degraded visual vigilance and memory for words, and time-of-day fluctuations were found in choice reaction time, logical reasoning, and word memory. Exercise also seemed to have an effect in that brain function of non-exercising subjects degraded sooner than they did for exercising subjects. So, sleep-deprived couch potatoes beware!
Researchers have found that people who stay up all night after learning and practicing a new task show little improvement in their performance. No amount of sleep on following nights can make up for the toll taken by the initial all-nighter.
Robert Stickgold and colleagues at Harvard Medical School report that people who learned a particular task did not improve their performance when tested later the same day but did improve after a night of sleep. To see whether the night of sleep actually caused the improvement, Stickgold trained 24 subjects in the same visual discrimination task, which consisted of identifying the orientation of three diagonal bars flashed for a sixtieth of a second on the lower left quadrant of a computer screen full of horizontal stripes. Half of the subjects went to sleep that night, while the other half were kept awake until the second night of the study. Both groups were allowed to sleep on the second and third nights. On the fourth day, both groups were tested on the visual discrimination task. Those who slept the first night identified the correct orientation of the diagonal bars much more rapidly than they had the first day. The other group showed no improvement, despite the two nights of catch-up sleep.
Another compelling study for the role of sleep on memory consolidation was published by Sean Drummond and his colleagues at San Diego State University and the University of California, San Diego. They combined memory performance with magnetic resonance imaging (MRI) to study sleep deprivation effects on verbal learning of young, healthy adults. After a sleepless night, free recall fell by about half, and the brain imaging analysis showed reduced blood oxygen activity in the temporal area. However, the areas of the prefrontal cortex that had been activated during remembering after normal sleep worked even more after sleep deprivation. What’s more, the bilateral parietal lobes and two additional areas in the prefrontal cortex, usually not activated after normal sleep, became active.
What about a small degree of sleep loss? A University of Pennsylvania study showed even a little sleep loss can devastate memory. People were assigned to sleep regimens of four, six, or eight hours of sleep each night for two weeks and tested periodically during the daytime for mental performance. Subjects who got four or even six hours of sleep performed as poorly on brain function tests as they did when kept from sleeping at all for three consecutive days. So, short-changing your sleep each night by an hour or so builds up a sleep debt that affects attention and working memory. In the study, performance decline was cumulative. An interesting aside from the study was that none of the 48 people in the study realized that their mental performance had deteriorated from the mild sleep loss. As a college professor, I wonder about the performance loss going on in students who short-change their sleep for months at a time.
There are also studies revealing lack of sleep BEFORE learning interferes with memory. Formally, this is called “proactive interference,” because it occurs in advance. The cause may relate to what was just explained: a sleepy brain doesn’t think effectively.
In another study, 28 healthy young adults were divided into two groups. On the first day, one group was kept awake for 35 straight hours. Participants in the other group spent a normal sleep night at home. At 6 PM of the next day, all subjects watched a slide show of 150 slides of landscapes, objects, and people who weren’t celebrities. All subjects then were sent home to have a normal night’s sleep. The next evening all subjects took a pop quiz on the slides, which were randomly mixed with 75 new slides. The test was for subjects to recognize whether they had seen each slide before.
Those subjects who had been sleep deprived on the first night scored the lowest, even though they later had a night to catch up on lost sleep. The upshot of it all is that lack of sleep is bad for remembering, whether the sleep loss occurs before or after learning events. For those who wonder why humans need to sleep, one obvious benefit is to enhance learning.
Need to learn something quickly? Take a nap. Daytime naps are said to rejuvenate energy and lower stress. Now there is evidence that naps speed up consolidation of memories.
Matthew Walker reports experiments showing nap enhancement of memory. In his study, 39 young adults were divided into two groups. At noon, all the participants took part in a memory exercise that required them to remember faces and link them with names. Then the subjects took part in another memory exercise at 6 p.m., after 20 subjects had napped for 100 minutes during the break. Those who remained awake performed about 10 percent worse on the tests than those who napped. Students take note: 10% is often the difference between an A and a B.
References: (1) Drumond, Sean, Brown, Gregory, G., Gillin, J. Chrisstian, Stricker, John. L. (2000). Altered brain response to verbal learning following sleep deprivation. Nature 403(6770):655-7. DOI: 10.1038/35001068 (2) Stickgold, R., James, L, and Hobson, J. (2000) Nature Neuroscience. 3 (12), 1237-1238. DOI:10.1038/81756 (3) Van Dongen, H.P.A., Rogers, N.L. & Dinges, D.F. Sleep debt: Theoretical and empirical issues. Sleep Biol. Rhythms 1, 5–13 (2003). https://doi.org/10.1046/j.1446-9235.2003.00006.x (4) Walker, Matthew (2010). American Association for the Advancement of Science annual meeting presentation, San Diego, Feb. 21, 2010.o verbal learning following sleep deprivation. Nature 403(6770):655-7.
Copyright of this article totally belongs to our author William Klemm, who is a senior professor of Neuroscience at Texas A&M University. This article is republished here from psychology today under common creative licenses