When I entered my third year of medical school, I quickly learned how important sleep is for memory. The year started out on the surgical wards of the George Washington University Hospital, and the schedule was punishing. The workweek started at 8 a.m. on Monday. We worked all day Monday, Monday night, and all day Tuesday, before finally going home Tuesday evening—a shift lasting more than thirty-two hours at a stretch. Then Wednesday was a normal workday of eight or ten hours. On Thursday it started again—working all day Thursday, Thursday night, and Friday, before collapsing at home on Friday night. We worked weekends, too. The schedule went on and on, seven days a week. Students, interns, residents—we all lived that way.
Reading this, you are no doubt horrified that clinical care would be in the hands of chronically sleep-deprived people. Luckily, things have changed a lot since then.
After just a few days on this schedule, I discovered that my short-term memory had become a sieve. Medication orders I had to write, patients I needed to see, lab results that needed to be checked—unless I wrote down everything, it all tended to slip away. The other medical trainees experienced exactly the same thing. With long sleepless stretches, our memory capacity was shot. Then, when we finally got a vacation and started to catch up on sleep, our memories returned to normal.
Sleep is important. As you doze off to sleep and your brain no longer has to pay attention to your conscious life, it can file away the experiences of the day, reset your emotional balance, shore up your pain control, and generally tidy up. At night, your brain is like a road crew that comes out on the highway after dark, setting out orange cones, filling potholes, painting lines, repaving, and then disappearing before the morning rush hour begins. If you keep going twenty-four hours a day, your potholes never get a chance to be filled in.
If it surprises you that your brain needs rest, think about all the other parts of your body. You can’t push your muscles twenty-four hours a day. After physical exercise, they need rest in order to repair and recharge. Every athlete knows that giving your muscles and joints time to recover and rebuild after exercise is as important as exercise itself.
Brain researchers say that sleep helps the brain consolidate memories. Let’s say that you’ve learned some new facts from a book or perhaps you gained some new skills on a musical instrument. Those memory traces are fragile at first and can easily be lost. Sleep helps them become permanent.
Research suggests that the first half of the night is when we consolidate memories of facts and events. The second half of the night, when rapid eye movement (REM) sleep is more predominant, is when we integrate memories related to new skills and to emotions.1
In the first part of the night, your body greatly reduces its production of cortisol. Cortisol is a hormone best known for its role in stress. It signals you to be alert to dangers and gets you ready for fight or flight. And when your brain is trying to file things away, the last thing it needs is to be distracted with a cortisol-driven high alert. So your brain turns down the cortisol and gets on with the work at hand.
In experiments, researchers have infused cortisol intravenously in sleeping human volunteers during the early part of the night and have found that it severely reduces their ability to retain memories for facts and events.1
So a medical student who, at one o’clock in the morning, is drawing a blood sample from a patient, then running the blood tubes to the laboratory, then wheeling a patient from the emergency room to a hospital bed, then consoling the family of a trauma victim and doing all the things medical students do is not only neglecting his or her need for sleep. That student is under stress, which stimulates the release of cortisol—the very hormone that interferes with memory consolidation when the brain is asleep. Even if the student could take a few minutes to doze off while the lab runs a test or a patient is having an X-ray, that sleep will not be of much use.
It is not just medical students who have this problem. Accountants, teachers, truck drivers, overworked parents, factory workers on swing shifts, members of Congress, and just about everyone else is prone to burning their candles at both ends, boosting their cortisol levels and interfering with memory.
In the second half of the night, everything changes. Rapid eye movement is a sign that you are dreaming. The facts and details that were important in the first half of the night now give way to the absurd dramas of dreams, infused with emotions of all kinds. And now cortisol levels start to rise. Some have suggested that just as cortisol interferes with memory consolidation early in sleep, it also softens emotional memories later in the night, so they are less intense and more manageable.
People with post-traumatic stress disorder get none of the softening of unpleasant memories that most of us thankfully experience. They repeatedly reexperience terrifying memories of traumatic events. Blood tests show that for some reason, they do not get the same cortisol rise that other people get in the second half of the night.1
Sleep may play an even bigger role in preserving your memory, a research team at Washington University in St. Louis has found. The researchers collected the cerebrospinal fluid from volunteers, using tiny tubes passed into their lower backs.2 Every hour for thirty-six hours, the researchers took a small sample while the volunteers talked, watched television, ate, or slept. What they were looking for was amyloid.
As you know well by now, the microscopic amyloid plaques that develop in the brain have been under intense study by researchers who are trying to understand why they form and how they contribute to memory loss. Amyloid is produced by brain cells, and eventually it flows down the spinal column, which is where the researchers were able to measure it.
The research team noticed several important things. First of all, amyloid has a circadian rhythm. That is, it rises and falls like a tide, day after day. A surge in amyloid production is followed by a prolonged lull, and then the cycle begins again, day after day.
The question, then, is what drives this cycle? If we could find out what causes the ebb and flow of amyloid, maybe we could have less of it, and reduce the brain-damaging effects it seems to have.
Maybe amyloid production rises at mealtime. Or maybe it’s physical activity that causes a rise in amyloid that then slacks off when we settle down. Or perhaps it’s just a day-night cycle, as is the case for many other hormones.
So the St. Louis researchers set up video cameras to watch what people were doing and simultaneously measured the amyloid in their cerebrospinal fluid. They watched them eating, talking, reading, sleeping, watching television, typing on their computers, and even using the bathroom. They tracked their body positions—flat, upright, or in between. And they looked at whether the amount of amyloid in their cerebrospinal fluid rose or fell with each activity.
Here is what they found: It did not matter if people were sitting, standing, or lying down. It did not matter if they were quietly reading, as opposed to walking around the room. It did not make much difference if they were sitting with their eyes closed versus having their brains get intensive stimulation from a television program or video game.
What mattered was whether they were awake. Amyloid goes up while you are awake and it falls when you go to sleep. The research team speculated that during sleep, the brain is able to clear itself of amyloid.
So this suggests that you have some control over your brain’s amyloid production. If you’re burning the midnight oil over a work project, watching the late show, or having a late-night snack, your brain stays up with you, cranking out amyloid all the while, or so it seems. If instead you go to sleep, your amyloid factories can finally turn out the lights, too. The sooner you go to sleep, the quicker they can close down. Sleep helps you shut off amyloid.
The St. Louis team noticed something else. Sleep seems less effective at shutting off amyloid production in older people compared with younger people. But the benefit of sleep never seems to be gone. The moral of the story is that sleep deprivation is among the worst things you can inflict on your brain. Those neurons are eager for you to get plenty of shut-eye.
As you read this text, please look up and check the clock. If it is after 10 p.m., close this book and go to bed. Your brain is tired, and you need to stop pushing it to create amyloid.
Sleep brings you one more benefit. When you’re unconscious, you can’t reach for a doughnut. And, yes, people who turn out the light and go to sleep early are thinner than people who stay up late. That’s good for your waistline, and, in turn, a trim waistline is good for health overall and for brain health in particular.
Sleep is your brain’s best friend. But many people have trouble getting a good night’s sleep. They lie down but just can’t seem to doze off. Or they wake up early, unable to return to sleep. Or perhaps they don’t give themselves the chance to sleep, staying awake till all hours, and setting the alarm to go off long before they have had a full night’s sleep.
If sleep is a challenge for you, let me offer some important tips, starting with a look at a few seemingly innocuous things that can be sleep destroyers:
Caffeine. If caffeine does not interfere with your sleep, you feel mentally fine, and your memory is sharp, don’t worry about it. The health issues related to caffeine are pretty minor. In fact, some evidence suggests that vigorous coffee drinkers actually have less risk of Alzheimer’s disease, as we saw in chapter 4. But if you’re tossing and turning, it is time to own up to what caffeine does.
In a word, it persists. If you have a cup of coffee at 8 a.m., a quarter of its caffeine is still circulating in your bloodstream at 8 p.m.
If you like technical explanations, the half-life of caffeine—the time it takes for your body to eliminate half the caffeine in your bloodstream—is about six hours. So the caffeine in your 8 a.m. cup of coffee is half gone by 2 p.m., and three-quarters gone by 8 p.m. But that remaining quarter-cup’s worth of caffeine can be enough to keep you from sliding into a deep and restful sleep.
And, worse, electroencephalograph (EEG) tests show that caffeine decreases the slow-wave sleep that is essential for memory consolidation in the early part of the night. So you might learn all kinds of things during the day, only to have them vanish in the night, as caffeine prevents your brain from making these fragile traces permanent.
“But I feel better when I have a cup of coffee,” you say. “And I feel terrible without it.” Of course you do. Many coffee drinkers feel very much that way. But caffeine is a complex drug. First of all, it is a mild painkiller, which is why it is added to many over-the-counter analgesics. It is also a stimulant. When your brain is habituated to these effects, you really will feel horrible when you miss your dose. You are in withdrawal.
And caffeine’s effects on your mental clarity are something else. It may help you stay awake and may take the edge off your aches and pains, but that does not mean your thinking is clear, your emotions are balanced, or your outlook is bright. For some people, caffeine keeps them awake, but mentally dull, fuzzy, or on edge.
Now, everyone is different. Some people metabolize caffeine more quickly, while others eliminate it more slowly. It pays to notice how caffeine affects you.
If you do break a caffeine habit, you may notice that your thinking becomes noticeably clearer, your personality becomes brighter and more flexible, irritability subsides, and the world feels like a better place to live. Before you get there, you will go through a few days of withdrawal as caffeine makes its last fading pleas for your loyalty, but you’ll very likely be glad you left it behind.
Alcohol. Alcohol is a bit of a devil. A drink or two gently lulls you to sleep. But in the early morning hours, your sleep starts to lighten. Around 4 a.m., worries wake you up, and you find yourself poring over problems left over from the previous day.
This early-morning awakening is not caused by mixing red wine with white or by the sediment in dark beer. It is caused by alcohol molecules transforming into closely related chemicals, called aldehydes. They are stimulants. These maladjusted chemicals have none of alcohol’s charms and instead make your sleep rocky.
Protein. Protein in the morning helps you stay alert. It does this by blocking your brain’s ability to produce serotonin, the mood-regulating chemical that also helps you sleep. So if your breakfast included some beans or soy products, such as veggie sausage, veggie bacon, or scrambled tofu, you’ll feel noticeably more alert than if you just had, say, a bagel. If you have your high-protein food first, and a starchier food second, you’ll be fine. Notice that I have mentioned vegetarian versions of high-protein foods. You do not want to do yourself the harm of including bacon, sausage, eggs, or other fatty, high-cholesterol foods in your routine.
At night, you want the opposite effect. That is, you want your brain to make serotonin to help you calm down and go to sleep. So let your dinner include more starches, and avoid high-protein foods. Natural starches, such as rice, pasta, and bread, stimulate serotonin production in your brain. You’ll find it easier to doze off and to stay asleep.
The call of the bathroom. If you’re getting up several times a night for a trip to the bathroom, you’re interrupting your brain’s nighttime routine. It might pay to drink a bit less water in the evening.
If those seductive bubbles in your sparkling water are making you drink more than you need, your body soaks that water up like a sponge, only to release it later on. Switch to still water and you’ll be less likely to overdo it.
You also may benefit from cutting down on salty foods. Salt holds water in your bloodstream and body tissues, and as the night progresses, that water gradually finds its way through your kidneys and into your bladder, waking you up.
If you’re in the bathroom several times a night because of prostate problems, it’s time for a good medical evaluation and perhaps treatment. And some evidence suggests that a plant-based diet—the same one that is good for your heart and helps prevent strokes and Alzheimer’s disease—is good for your prostate, too.
Lack of exercise. If you have plenty of physical activity during the day, you will sleep better at night. Think about children. They run around so much all day that they are practically comatose at night. As we get older, we tend to be less physically active. For some of us, the only exercise we get is the tips of our fingers typing on a keyboard. So when we lie down to sleep, our brains might feel tired, but our muscles are not, so they do not demand sleep. Your sleep will then be light and easily interrupted by the slightest noise or worry. And you will end up dragging through the day.
So to get a good night’s sleep, get a little exercise. The best exercises for a good night’s sleep are those that put a bit of strain on your muscles—push-ups or weight lifting, for example. Even a little exercise can have a noticeable effect on sleep.
It goes without saying that you should talk with your doctor before starting a new exercise program. You’ll want to be sure your heart and joints, in particular, are ready for it.
A pre-bedtime routine. If you have a cat, watch what your cat does before going to sleep. She’ll stretch out her paws, give a big yawn, and then curl up and go to sleep. Dogs do the same. In medical school, I had a pet rat who’d been saved from a laboratory, and she used to stretch out her cute little rat paw, give a yawn, and dream about trigonometry or the rings of Saturn, or whatever rats dream about.
People do the same. Children stretch and yawn, getting ready for sleep. But have you noticed that adults often do not go through this ritual? Whether it is due to caffeine, stress, or something else, they simply close their magazine, turn out the light, and hope to doze off.
If you had imagined that these presleep rituals have no function, try a little test. About a half-hour before bedtime, open your mouth to simulate a big yawn. Reach out your arms and give them a good stretch. At first you’re just going through the motions. But do this four times—a big yawn and a stretch each time. You’ll soon trigger a genuine yawn and a deep-muscle stretch. Then notice what this does to the quality of the sleep that follows. What you will likely discover is that something about stretching and yawning prepares the body and brain for sleep.
If you are having trouble sleeping, use this simple routine, and see if it helps.
Some people are afraid to nap during the day for fear they won’t sleep at night. That’s possible, if it is a very long nap. But a short nap will not interfere with sleep and might actually help you get rid of accumulated tension so you’ll unwind more easily at night.
A man in Virginia woke up to the sound of his alarm and went down to the kitchen to fix his breakfast. And there on the kitchen table was an open box of cereal, a selection of fruit, a loaf of bread, and a variety of other groceries. “That’s strange,” he thought. “Who put those there?” He looked in his refrigerator and found it surprisingly well stocked with orange juice, cartons of almond milk, salad ingredients, and many other things. Since he lived alone, these magically appearing groceries were a puzzle.
The night before, he had taken zolpidem, the massively popular sleeping pill, marketed under the name Ambien. Under its influence, he had driven to the store and stocked up for the week, and all memories of the shopping trip were wiped out before morning. Many other people have reported similar bizarre experiences of sleepwalking, sleep-driving, or binge-eating, with subsequent amnesia for the impromptu party they had put on for themselves the night before.
Some sleep experts swear by Ambien, saying it is much safer than many older sleep aids. But the fact is, memory problems are very common, and Ambien’s prescribing information has been updated to include stern warnings about people doing all sorts of other things they cannot remember later. I strongly suggest you avoid it.
Other sleeping medications can affect memory, too. Some of them block the neurotransmitter acetylcholine. On this list are diphenhydramine (Benadryl, Sominex) and doxylamine, which is found not only in sleeping medications (such as Unisom) but also in nighttime cold remedies (such as NyQuil, Alka-Seltzer Plus Night Cold, and Tylenol Flu Nighttime). If you block acetylcholine too strongly—for example by taking too large of a dose or by combining two or more medicines with similar effects—you can end up with dry mouth, blurred vision, constipation, urinary retention, and, eventually, confusion and memory problems.
Many antidepressants are used for sleep, too, and their tendency to block acetylcholine can cause the same problem. We’ll have more to say about medications in chapter 8.
Sleep is essential for memory and many other aspects of health. I would encourage you to turn out the lights at 10 p.m. and let your brain get on with the work of repairing and rebuilding.
Be careful about caffeine and alcohol, skip high-protein foods late in the day, get plenty of exercise, and try the pre-sleep routines I mentioned. Your brain will thank you for it.