Why We Forget and How to Remember Better, page 24
Stages of Sleep
Matt Walker, at the University of California at Berkeley, helped to both demonstrate the importance of sleep for new learning and provide evidence for one part of its mechanism. He invited college students into his laboratory and, at noon, had them all study 100 faces, each of which was paired with a unique name. Half of the students were then randomly chosen to nap, while the other half stayed awake. At 6 p.m., everyone tried to learn another 100 face–name pairs. Despite equal ability to pay attention, the group that napped learned 20% more pairs compared to the group that stayed awake. Moreover, this increased capacity for learning correlated with a specific stage of sleep. Which stage is most important? To answer this question, we need to first explain the different stages of sleep.2
As you know from your own experience, if you are woken up at night from a noise or your bladder calling, it is usually when you are sleeping lightly or perhaps dreaming, and only rarely is it when you are in a “deep” sleep when you feel dead tired. Perhaps you’ve only experienced waking from a deep sleep when you are jet-lagged. In any event, you probably have an intuitive experience that your sleep occurs in different stages. In fact, we generally cycle through different stages of sleep about five times each night.
The stages are named based upon whether your eyes are moving rapidly or not and also what can be seen on an electroencephalogram (EEG) if you have electrodes on your head recording your brain activity while you are sleeping. Rapid-eye-movement (REM) sleep is the stage when you have your most vigorous—and often bizarre—dreams. Non-REM (NREM) sleep is divided into stages 1, 2, 3, and 4. Stages 3 and 4 are usually grouped together and referred to as “slow-wave sleep,” because during those stages your brain cells are firing in synchronous waves, creating “slow waves” on the EEG. Slow-wave sleep is “deep” sleep when you are hard to awaken, NREM stage 2 is “lighter” because you are easier to waken, and stage 1 is lighter still—so light that you may not even realize you were asleep if awakened from it.
Figure 20.1 (bottom) shows some typical sleep cycles across a normal night of sleep. You can see how the proportion of time spent in different cycles varies across the night. Note also that it is normal to briefly wake a couple of times each night, although you may not recall these awakenings.
Returning to Matt Walker’s experiment with college students learning face–name pairs, he found that when the students were napping it was specifically the amount of time they spent in NREM stage 2 that was beneficial for learning more pairs. NREM stage 2 sleep contains bursts of activity called sleep “spindles” because of how they look on EEG. In fact, the number of sleep spindles themselves correlated with students’ ability to learn more face–name pairs. This finding and others like it suggest that these spindles may help memories to be processed so as to avoid them from being forgotten.
Consolidate Memories While You Sleep
You now understand another reason that sleep is so important to memory—it allows you to consolidate your memories, transferring them from transiently-accessible memories to those that can be recalled days or even years later. As we discussed in Chapters 4 and 5, many episodic memories that represent events of your life will still have a connection to the hippocampus, whereas most semantic memories for facts and information will be stored in the cortex alone.
This sleep-dependent consolidation process doesn’t only free up the hippocampus for additional new learning but also helps to solidify the information you’ve learned in your memory. For example, if you learn a list of facts in the evening and then sleep for 8 hours, you could remember 20% to 40% more of them than if you learned that list in the morning and were awake for 8 hours. Moreover, the number of facts you will remember overnight correlates with your total time in NREM sleep.3 Reviewing information you’re hoping to commit to memory at night, soon before you sleep, can be a powerful tool to help you retain newly-learned information.
Your procedural memories—your memory for learned skills such as basketball and skiing—also become consolidated when you sleep. While you sleep, your brain is rehearsing the sequence of motor commands needed to perform an action. Sometimes, this means that you can wake up better at a motor skill than you were when you went to sleep. As we discussed in Chapter 2, although this “offline” learning can occur while you are awake, it is more robust when you are sleeping—and in particular, when you are in stage 2 NREM sleep. Because stage 2 NREM sleep is most prevalent at the end of sleep, athletes may not be helping themselves when they get up early to practice if to do so they are reducing their sleep by an hour or more. Basketball player LeBron James, for example, understands the importance of sleep to his performance: He sleeps 8 to 9 hours at night and may nap throughout the day for a few more hours.
Sleep to Remember What Is Important
Sleep doesn’t indiscriminately retain all memories—NREM sleep preferentially boosts the long-term memories that your brain has tagged as important in some way. Memories that are not important—such as what you had for breakfast this morning—will not be strengthened and, after a few nights of sleep, are likely to be forgotten altogether. Return to Parts 1 and 2 for why some memories are tagged as important and some are not.
Can You Use Sleep to Enhance Particular Memories?
If NREM sleep is so beneficial for memory, you may wonder if there are any ways to enhance some of the particular content you learned that day while you sleep.
Ken Paller and his colleagues at Northwestern University tested this idea by teaching participants to associate 50 unique object images with 50 specific locations on a computer screen. Each image was paired with its related sound, such as meow with cat. Participants then napped while listening to 25 of the paired sounds during NREM sleep. These sounds were played quite softly, to avoid awakening the participants. After the nap, participants were asked to use the computer mouse to move each object image to its specific location on the computer screen that they learned prior to their nap. Ten of 12 participants showed more accurate placement for the objects whose sound was presented while sleeping compared with the non-presented objects. Note that participants were unaware that they were presented with sounds while they were napping and were, in fact, at chance when asked to guess which sounds were presented while they slept.4 There have now been over 90 experiments with more than 2,000 subjects confirming that replaying cues during both stage 2 NREM and slow-wave sleep works to improve memory for the cued material, including odors, sounds, and vocabulary words.5
So, should you start recording information you want to remember better and play it back while you are sleeping? Or should you try to enhance your memory more generally for information that was learned while listening to specific music or smelling specific odors by having that music or odor with you while you are sleeping? Perhaps. Experiments looking at the practical application of these types of techniques are currently ongoing and may lead to new ways to boost your memory.
Interconnecting Memories: REM Sleep
You may have noticed that, thus far, we have mainly discussed the role of NREM sleep in memory. But REM sleep is important too. One intriguing hypothesis suggests that it is during REM sleep when your newly consolidated memories become interconnected with all of your prior memories, including your autobiographical life history and your storehouse of facts and knowledge.
If we ask you to think of the first word that comes to mind when we say “dream” you might think “sleep,” and if we say “doctor” you might think “nurse,” as sleep and nurse are strong associations for dream and doctor, respectively. But when Robert Stickgold and colleagues at Harvard University woke participants from REM sleep, they found that—judging by how quickly participants responded to different word pairs—weak associations were actually more accessible than strong associations. Participants awakened from REM were more likely to associate words with weak associations—such as “dream” with “sweet” and “doctor” with “office”—than they were to associate words with strong associations. It’s as if, during REM sleep, the brain has an opportunity to try out different associations, and to see if any of them provide useful new insights. Often, they don’t—although they may lead to strange dreams. But, as we talked about in Chapter 10, the ability to notice associations between different memories is critical for making new inferences, and so, sometimes, these weak connections highlighted in REM sleep provide an important new perspective on a problem.
Following up on these ideas, Kristin Sanders and colleagues at Northwestern University found that when participants were presented with puzzles in the evening, 20% of the unsolved puzzles could be solved the next morning. Although we consider this percentage by itself an impressive display of the power of sleep, participants were able to solve an additional 10% of unsolved puzzles if they were cued by being exposed to a sound related to the puzzle while they slept.6 This experiment demonstrates not only the ability of sleep to help you solve problems, but also the importance of memory reactivation in finding the solution.
The most cited historical example of such creative problem-solving work while sleeping is that of Dmitri Mendeleev, who dreamed on February 17, 1869, where all the atomic elements should be placed in the periodic table—something he was unable to do for months while awake. Writing them down as soon as he awoke, he only needed to make one correction to the solution provided by his dream. Perhaps you, too, have been puzzling over a problem that your waking brain has been struggling to solve. As you are lying down with your eyes closed preparing yourself for sleep, spend a minute or two thinking about your problem. You just might wake up the next morning with the answer.
You’ll Feel Better in the Morning
Have you had the experience of being upset, and someone tells you, “Don’t worry, you’ll feel better in the morning”? When you woke the next day, did you feel better? We bet you did. In fact, we would guess that you continued to feel a little bit better each morning. Your experience of “feeling better” is related to a few functions of sleep. One is as we just described: Sleep can help you to gain a new perspective on a problem. You may fall asleep dreading a conversation you need to have with a coworker the next day. But in the morning, you may realize that you can use an experience from years ago—when you were on the receiving end of a critique from a boss—to help you constructively guide the conversation.
Another important function of sleep is to strip off the emotions related to painful, upsetting memories, while still keeping the memory content. In this way you still very clearly remember what upset you but, with sleep, you don’t re-experience the full emotional intensity of the event every time you retrieve it from memory.
We again turn to a study by Matt Walker to help us understand how this stripping away of emotions occurs while you are sleeping. He had individuals view emotional photos while in an MRI scanner to monitor their brain activity. As expected, there was strong activation in the amygdala, an almond-shaped structure just in front of the hippocampus that generates emotions. All participants returned to the scanner 12 hours later, but half of the subjects were initially tested at night and therefore had a night’s sleep between sessions, whereas the other half were initially tested in the morning and so were awake between the sessions. The researchers found that those who slept—but not those who were awake—had significant reductions in their emotional feelings as well as amygdala reactivation. Moreover, these important reductions correlated with the amount of time spent in REM sleep, suggesting that this is another important function of this stage of sleep.3
A related study by Rick Wassing and colleagues in Amsterdam found that not only is REM sleep important for this amygdala adaptation, but that the REM sleep needs to be relatively continuous with few interruptions for the adaptation to be successful. Participants whose REM sleep was highly fragmented did not show the beneficial emotion-reducing effects of sleep.7
Another researcher, Rosalind Cartwright at Rush University, found that it was critical for the dream itself to relate to the painful experience in order for individuals with depression to strip off the painful emotions and get past those experiences. Individuals who did not dream about their painful, traumatic experience could not remove the emotions from the event and did not experience remission of their depression.8
You may recall circumstances in which you dreamed—perhaps for several nights—about an upsetting event, only to feel somewhat better about it in the days and weeks that followed. Andrew has had this experience related to the most painful and traumatic experience of his life, that of his son Danny being diagnosed with severe autism before age 2. Danny never learned to talk. And so, the dreams that Andrew had, with variations for weeks and months, involved Danny talking. Andrew still feels sad on occasion when thinking of what might have been without the autism. But, likely because of these dreams, Andrew has been able to move past the pain of Danny’s limitations and enjoy Danny’s strengths and the time they spend together.
Sleep to Reduce Your Risk of Alzheimer’s Disease
In Chapter 13 we mentioned how Alzheimer’s disease is caused by amyloid plaques and neurofibrillary tangles, collections of proteins that can be seen under the microscope. Most researchers believe that the disease starts with the amyloid plaques. Then, when the plaques get large enough, they begin to damage neighboring brain cells, which then form tangles—killing the cells.
It turns out that we all make some of this amyloid protein during the day. The normal function of amyloid is not clear; some researchers (including Andrew) believe it is involved in the brain’s defense against infections. Although some amyloid may therefore be beneficial, too much is thought to directly cause Alzheimer’s disease. How does our body get rid of excess amyloid so we don’t all develop Alzheimer’s? By sleeping, of course.
Maiken Nedergaard at the University of Rochester has done pioneering work on the brain’s drainage mechanism, called the “glymphatic system.” Although this system is active during the day, it is 10 to 20 times more active at night. So, we need our sleep to clear away excess amyloid protein.
As you might expect, there are studies that show you are at increased risk of dementia and Alzheimer’s disease if you sleep poorly.910 But there are also studies that show you can reduce your risk of Alzheimer’s disease by improving your sleep.11
Sleep to Reduce Your Risks of Strokes and Vascular Dementia
After Alzheimer’s disease, strokes are the most common cause of memory loss and dementia. Insufficient sleep increases your risk of weight gain, high blood pressure, diabetes, and heart disease—all of which increase your risk of strokes, memory loss, and dementia, as discussed in more detail in Chapters 13 and 14.
Don’t Pull an All-Nighter
It should be obvious by now that sleep is so important for memory and your brain that you’re only going to decrease your performance on your test the next day if you “pull an all-nighter” and stay up all night. In addition to being tired the next day, leading to difficulty paying attention, when you are sleep deprived you’ll retain less of the information you learned the previous day and will be much less able to learn new facts compared to having a good night’s sleep.
Can you just catch up on your sleep on the weekend? No. Sleep isn’t like a bank that you can borrow from and pay it back later. Of course, if life circumstances prevent you from getting enough sleep during the week (perhaps you are a new parent or working multiple shifts), then—by all means—take a nap when you can and try to go to sleep a bit earlier on the nights when it’s possible for you to do so. There is some evidence that sleeping a little extra on the day prior to a late night can help your brain to function better. But, for ideal memory function, it is important to prioritize sleeping soundly each night.12
Sleep and School
Now there are some individuals who routinely flout healthy sleep guidelines: They stay up late, get up early, and try to catch up on their sleep during the weekends. Who are these dangerous rulebreakers? Students, of course—particularly teenagers.
Although we’ve introduced this topic in a humorous way, student sleep is no laughing matter. Sleep has been studied in students in elementary school, middle school, high school, college, and medical school. In each and every case, the students who slept better performed better in school academically.1314
Importantly, one study from researchers at the Massachusetts Institute of Technology measured sleep and test performance in college students during the month, week, and night prior to a test. Interestingly, although they found no relation between test performance and sleep the night before a test, they found a strong relation between sleep quality and test performance in the month and week before. This beneficial effect of sleep was so strong that it accounted for almost one-quarter of the differences observed between the grades in the class.15 This and other studies like it make the point that good sleep habits over time really do make a difference in learning.
Speaking of school, students perform remarkably better when school start times are pushed back from 7:30 to 8:30 a.m. or later. Adolescence is a period of development associated with a circadian shift toward later sleep-onset times and later awakenings, and so shifting school start times can bring them into better alignment with teenagers’ natural circadian rhythms. In one example from Edina, Minnesota, after shifting from a 7:25 a.m. to an 8:30 a.m. start time, verbal and mathematics Scholastic Aptitude Test (SAT) scores increased from 605 and 683 to 761 and 739, respectively.3 Petition your school district today for more natural start times.
