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Matthew Walker is director of the Sleep and Neuroimaging Lab at Beth Israel Deaconess Medical Center and Assistant Professor of Psychology at Harvard Medical School. He investigates the impact of sleep on memory both at the behavioral and neuroanatomical levels. His research has shown how getting sleep after learning motor-dependent tasks (which involve so-called "procedural memories") improves performance on these tasks. Walker has also studied the sleep-dependent nature of memories that can be expressed verbally ("declarative memories"), in particular memories with emotional content. Using functional magnetic resonance imaging (fMRI) to image the brain, he has explored how memories may be reorganized in different regions while we sleep, thus consolidating and enhancing them. |
On July 16, 2007, Matt Walker (see bio below left) answered selected viewer questions about sleep and memory. Please note we are no longer accepting questions, but see The Sleep-Memory Connection and our links and books section for additional information. Q: Is there a good answer as to why we don't need as much sleep as we age? Could this be a reason for mental decline? A: Dear Claudia, Fantastic question! We certainly know that the cognitive hallmark of aging is a progressive decrease in the ability to acquire and retain new facts. Furthermore, a physiological signature of aging is reduced and fragmented sleep, often involving an inability to fall asleep quickly, getting up throughout the night, and early morning awakenings. Considering what we now understand about the role of sleep in memory processing, when you put these two facts together, they certainly tell a suggestive story. Surprisingly, we do not yet know if poor sleep and poor memory in aging are causally related or simply correlated. Interestingly, though, the type of sleep that suffers the most dramatic decline with age is deep non-REM sleep (stages 3 and 4, also known as "slow-wave sleep"), and this type of sleep is associated with the consolidation of newly learned facts and experiences. I am sure the answers to questions like yours will emerge in the coming decade, and if it is true that reduced and fragmented sleep does contribute to mental decline, such sleep abnormalities are potentially treatable. This might then offer the opportunity for improved cognitive function in the elderly. [Editor's note: For a description of the various REM and non-REM stages of sleep, see The Sleep-Memory Connection.] Q: Would sleep apnea cause memory problems? What "type" of sleep (e.g., REM) would a person miss if he or she had sleep apnea? A: Hello Heather, Sleep apnea (which actually translates to "absence of breath") is characterized by frequent pauses in breathing during the night (due to a variety of causes) and is associated with a reduction in blood oxygenation as a consequence. Depending on the cause, sleep apnea can affect both deep non-REM sleep (stages 3 and 4) as well as REM sleep. More generally, apnea also reduces the overall efficiency of sleep due to the brain's response to breathing cessation, which is to briefly wake up (although few people consciously perceive or recall these awakenings). Regarding the specific consequence to memory, surprisingly little research has focused on this question, despite the ramifications. But we are starting to make progress. For example, a recent preliminary study demonstrated that the brains of sleep apnea patients, particularly the frontal lobes, had to work far harder to achieve a similar level of learning compared to the brains of a group of control subjects that did not suffer from apnea. This would indicate that, when suffering from sleep apnea, the human brain becomes less efficient and has to try harder when acquiring new information. Nevertheless, we are still only at an early stage of understanding the consequences of disrupted sleep on memory, since it is really only in the last decade that scientists have begun to understand the role of sleep in preparing the brain for next-day learning and, subsequently, consolidating that new information the following night. Q: Slightly tangential, but—I hope—related: Is there a known or studied connection between excessive daytime sleepiness, fatigue, etc. and attention deficit disorder? Thanks, A: Hello, The potential link between attention deficit disorder (ADD) and sleep is a very active research topic right now, and there still remains controversy over the interplay between these factors. Parents commonly report greater sleep problems in children with a diagnosis of ADD, relative to those without, suggesting that there may be some association (although not necessarily causal). This is complicated by more rigorous clinical studies that have found sleep abnormalities less consistently. Perhaps the most frequently identified sleep abnormality in ADD is nighttime leg movement issues, including limb restlessness. These symptoms point towards a central nervous system origin, possibly related to neurochemical changes within the brain. Despite these new lines of evidence, we still require largescale studies to tackle the question, and I suspect they are already under way. Q: What connection or influence does depression have with sleep or lack of sleep? A: Hi John, There are a variety of different types of depression, and the associated sleep impairments can be different amongst them. Certainly, a vast number of depression sufferers self-report having sleep difficulties, and clinically controlled sleep studies support these claims. A common finding has been alterations in REM sleep, specifically an increase in the amount of REM and its earlier arrival during the 90-minute sleep cycle. Interestingly, many of the older anti-depressant medications had the effect of reducing the amount of REM sleep, although more recent new anti-depressants do not have such dramatic REM effects. Another common finding in people with depression, particularly in men, is a reduction in the quality of deep non-REM sleep (stages 3 and 4), although it is important to note that the above changes also depend on other factors such as age. Nevertheless, these findings signal an intimate relationship between sleep and our mental and emotional well-being—a relationship we are continuing to learn more about. Q: How do "sleep aids," such as Ambien, change sleep patterns and memory? A: Hi Jean, Well, we know a lot about the first part of your question (sleep patterns) and very little about the second (memory). Ambien (Zolpidem) binds to specific receptors in the brain that commonly have the effect of "quieting down" neural activity, and as a consequence, the drug can help people who are suffering from insomnia by allowing a faster transition into sleep. The effects of drugs like Ambien, unlike earlier sleep medications of some decades ago, are more short lived (short-acting) and can prevent next-day grogginess from occurring. Numerous sleep studies of the effects of Ambien have been conducted. Common findings include an increase in deeper stages of non-REM sleep, and there may also be a decrease in REM sleep. How these changes alter memory function has yet to be examined. It is likely not going to be a simple story though, since different types of memories seem to rely on different types of sleep (see The Sleep-Memory Connection for more details). Thus, while one type of memory may benefit from an increase in non-REM sleep, another may suffer from a decrease of REM. Of course, eating a balanced diet, taking regular exercise, and refraining from excessive caffeine and alcohol intake (all the good things your doctor, and mother, advise!) can help produce healthy sleep patterns without necessitating sleep medication. Q: I read that we cycle [through the various stages of sleep] 4-6 times during the night. What is the reason behind this? A: Hi Crystal, This is one of the oldest and most fundamental questions surrounding sleep, and there is still no consensus as to the answer! The reason for this lack of consensus is probably because the varied stages of sleep evolved to support many different functions instead of just one, hence there is no simple answer; rather, there are many right answers. Adding to the complexity of the question, the 90-minute sleep cycle (in humans) of non-REM-to-REM is not balanced in composition as we continue through the night; the first 2-3 cycles contain much more deep non-REM sleep, while the remaining 2-3 cycles are comprised of more REM sleep. Some theories have suggested that the progressive increase in REM sleep (a brain state more similar to waking than deep sleep) as we get closer to morning allows our brain to start preparing to wake up—like a whale bobbing up to the surface for the intake of air. But this is not likely to be the only, or even primary, reason for the cerebral sleep-stage ballet we perform throughout the night. Indeed, in order to develop such complex sleep-stage patterns, evolution has had to put in place many intricate neural systems in the brain, and I can only imagine that this alternating flip-flop between different neurochemical cocktails at night serves more than just the purpose of preparing us to wake up in the morning. Instead, it is likely that these oscillating sleep cycles afford a multitude of functions for both brain and body. But sleep, with its fluctuating stages, remains the proverbial "riddle wrapped in a mystery inside an enigma." Q: Is it true that the hours of sleep before midnight are more important than the hours after midnight? A: Hi Mary, There is nothing particularly special about the clock time of midnight for sleep. Upon falling asleep, there is a general propensity to go through a 90-minute cycle of non-REM sleep followed by REM sleep. However, as mentioned above, the ratio of non-REM-to-REM sleep within these 90-minute cycles changes across the night, such that early in the night (e.g., 11p.m.-3a.m.), the majority of those cycles are comprised of deep non-REM sleep (stages 3 and 4) and very little REM sleep. As we push through to the second half of the night (e.g. 3a.m.-7a.m.), this balance changes, such that the 90-minute cycles are comprised of more REM sleep (the stage commonly associated with dreaming), together with a lighter form of non-REM sleep (stage 2). But there is a subtle twist to this story that rises above these 90-minute laws: the earlier in the night, the greater the propensity for deep non-REM sleep, and the later in the morning, the greater the propensity for REM. Therefore, someone who sleeps from 9p.m. to 5a.m. (8 hours total) will have a different overall composition of sleep—biased towards more non-REM—than someone who sleeps from 3a.m. to 11a.m. (also 8 hours total), who is likely to experience more REM. Indeed, if you afford yourself the luxury of sleeping in later during the weekends, you'll experience this phenomenon, with a greater likelihood of having more dreams due to the increased proportion of REM sleep. Q: For those of us who like to cram for exams, is it possible to learn while sleeping, e.g., by listening to audiotaped lectures while we sleep? A: Dear Don, A series of studies done in the early 1990s investigated whether information conveyed to the brain upon falling asleep and soon after was later remembered better. The answer was no. Indeed, when we are asleep, the brain stops focusing and perceiving external signals and, instead, focuses internally on what it learned during the day. As such, your brain's mode of operation during sleep is particularly unfavorable to learning from your surrounding environment (such as a tape player under your pillow). What does this mean for those who want to cram effectively for exams? The answer is a simple, three-part trick: (1) Get a full night of sleep before a day of cramming. This will allow your brain to be as receptive as possible to laying down new information (a dry sponge is a good analogy here, and if you don't sleep, it will still be water-logged with learning from the prior day). (2) During the following day, learn the material deeply (i.e., think about what you are learning, and understand it, rather than use simple rote memorization). (3) Get a full night of sleep afterwards to cement and consolidate that information into the architecture of the brain. Indeed, a recent study of our own demonstrated that, following learning, sleep not only enhances individual pieces of new information but also builds associative links between them, allowing you to "see the bigger picture." Good luck with your cramming and exams! Q: I've noticed that certain activities dramatically increase my ability to recall my dreams in vivid detail each day. If dreams play some role in the learning process, might a person's ability to remember their dreams affect it as well? A: Hello Nathaniel, There are numerous interesting aspects to this question. First, you can certainly do a lot to improve your recall of dreams in the morning. For example, keep a notepad and pen or tape recorder by your bed, and when you first wake up in the morning, don't sit up and get out of bed immediately. Instead, lie and think back over the dream and process it, then sit up and write the dream down (or dictate it). This may sound funny, but also saying a little mantra-chant before you go to bed at night, such as "I will remember my dreams, I will remember my dreams," often reminds people to remember their dreams. Second, there is currently little evidence to suggest that dreaming, or at least the extent of dream recall, plays any role in learning and memory processing. Instead, it may be that the brain state from which much of dreaming comes—REM sleep—is critical for memory processing, but dreaming itself is simply an epiphenomenon of REM. A good analogy is a light bulb. The function of a light bulb, and the machinery of which it is made, is to create light, just like one function of REM sleep, and the brain machinery that generates it, is to process memories. Yet a by-product of a light bulb is heat—it was never its functional purpose, it is simply a consequence of creating light in this way. Similarly, dreaming may be a by-product of REM sleep. This is not to say that evolution hasn't co-opted a function for dreaming (or may still), but it may not have been the reason REM sleep evolved. Third, while I and many other sleep scientists remain open to the possibility that dreaming is functional in some way, I suspect that remembering dreams is not a requirement—otherwise, dreams would be far easier to remember than they are. Indeed, I would even go so far as to theorize that it is actually beneficial not to remember our dreams. We know that many memories operate below the radar of consciousness, far more than operate above it. This may, in part, be a way for the brain to process information faster. It may be so for dreaming, too: if dreaming does assist in digesting information learned during the day, it does so without the requirement of remembering the dream. In other words, dreaming has done its covert job at night, covered its tracks, and you are better off for it the following day. Q: Research shows that students living in poverty get less sleep than students in higher socioeconomic groups. As a teacher of students living in poverty, I'm wondering if you can direct me toward research that is specific to how children learn and perform academically. A: Dear M., This is an important societal question. I'm not quite sure of the specific circumstances of poverty, but I would recommend reading Efficient Learning for the Poor: Insights From the Frontier of Cognitive Neuroscience by Helen Abadzi. Q: I have heard that the "perfect" length of a nap is 20 minutes. I have heard that this is so that you get some rest without entering the REM sleep stage. Is this true? Based on my personal experience, I think it may be. If I keep my nap short, I often wake up feeling refreshed, but if my nap is too long (but not long enough) I wake up feeling very groggy and tired. A: Hello David, The "perfect" nap depends on what one is trying to achieve. For example, my group and other researchers have found that naps of 90 minutes can have a beneficial influence on consolidating certain types of memories (those we call "procedural" or skill memories), but as the naps get shorter (between 30-60 minutes), the benefits are significantly less. Yet, for recovering from tiredness or fatigue (e.g., following a poor night of sleep), a 15-20 minute nap can have positive effects on alertness. Some studies have even shown that as little as five minutes of napping can have attention benefits. As you note, however, the deeper the brain goes into the non-REM/REM cycle, the longer it takes to get over the so-called "sleep inertia effect" upon awakening. More generally, your question raises the historical and evolutionary issue of how humans were originally designed to sleep. Several remote cultural tribes, untouched by industrial civilization, are known to sleep in two phases (called biphasic sleep), with approximately six hours of sleep at night and 1.5 hours of sleep in the afternoon. Similarly, European Mediterranean cultures are known for the practice of afternoon siestas. While this may, in part, be driven by external environmental temperature, it is also driven internally by our natural biological activity patterns (circadian rhythms), which drop around that post-lunchtime period (i.e., the proverbial head-nodding during afternoon lectures or meetings). If this biphasic model is indeed our naturalistic sleep pattern, rather than having one single bout during the night, we may choose to question the benefits of Thomas Edison's electric light bulb invention. (Edison, I may add, often took naps to gain creative insights and ideas!)
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