Biological Psychology Links

By ANAHAD O’CONNOR THE FACTS In the world of sleep research, dreams are something of a black box. But one tidbit that scientists have discerned is the peculiar but predictable pattern in which dreams tend to occur. Research suggests that much of what happens in a dream is unique to that dream. But some events from a person’s day can be incorporated into dreams in two stages. First there is the “day residue” stage, in which emotional events may work their way into a person’s dreams that night. But that is followed by the more mysterious “dream lag” effect, in which those events disappear from the dream landscape — often to be reincorporated roughly a week later. This lag has been documented in studies dating to the 1980s. A 2004 study in The Journal of Sleep Research began to shed some light on this cycle. Researchers reviewed the journals of 470 people who recorded their dreams over a week. The dream-lag effect was strongest among people who viewed their dreams as a chance for self-understanding; their dreams often involved the resolution of problems or emotions tied to relationships. The researchers speculated that the delayed dreams were the mind’s way of working through interpersonal difficulties and even “reformulating” negative memories into more positive ones. Other studies have also shown a connection between dreams and this type of emotional memory processing. THE BOTTOM LINE The dream cycle can be much longer than a single night. Copyright 2010 The New York Times Company

By Tina Hesman Saey Eating may rejuvenate a tired body, but new research in fruit flies suggests that fasting actually helps ward off the ravages of sleep deprivation. Starving sleep-deprived fruit flies sheltered the insects from sleepiness and fended off learning and memory difficulties associated with grogginess, researchers report August 31 in PLoS Biology. Starvation may slow down the buildup of sleep-inducing substances that accumulate while an animal is awake, says Paul Shaw, a neuroscientist at Washington University School of Medicine in St. Louis who led the work. The new study suggests that a rise in lipids, a type of fat, during wakefulness makes fruit flies sluggish. Learning how lipids induce sleepiness may eventually help develop new sleep remedies and shed new light on how sleep evolved. The findings herald “a big change for the field” of sleep research, says Robert Greene, a neurobiologist at the University of Texas Southwestern Medical Center at Dallas. “It emphasizes the importance of metabolism and its interaction with sleep.” Scientists are still debating why animals and people sleep (SN: 10/24/09, p. 16). To learn what happens during sleep, most researchers compare sleep-deprived animals with animals that have been allowed to sleep normally. In the new study, Shaw’s team took a different approach. The researchers wanted to see if there was a difference between fruit flies that have been kept up all night by bumping them awake whenever they tried to sleep and fruit flies that stay awake longer than normal because they are starving. © Society for Science & the Public 2000 - 2010

By MATTHEW PERRONE WASHINGTON — Andrew White returned from a nine-month tour in Iraq beset with signs of post-traumatic stress disorder: insomnia, nightmares, constant restlessness. Doctors tried to ease his symptoms using three psychiatric drugs, including a potent anti-psychotic called Seroquel. Thousands of soldiers suffering from PTSD have received the same medication over the last nine years, helping to make Seroquel one of the Veteran Affairs Department's top drug expenditures and the No. 5 best-selling drug in the nation. Several soldiers and veterans have died while taking the pills, raising concerns among some military families that the government is not being up front about the drug's risks. They want Congress to investigate. In White's case, the nightmares persisted. So doctors recommended progressively larger doses of Seroquel. At one point, the 23-year-old Marine corporal was prescribed more than 1,600 milligrams per day — more than double the maximum dose recommended for schizophrenia patients. A short time later, White died in his sleep. "He was told if he had trouble sleeping he could take another (Seroquel) pill," said his father, Stan White, a retired high school principal. An investigation by the Veterans Affairs Department concluded that White died from a rare drug interaction. He was also taking an antidepressant and an anti-anxiety pill, as well as a painkiller for which he did not have a prescription. Inspectors concluded he received the "standard of care" for his condition. Copyright 2010 The Associated Press.

By Nicholette Zeliadt Our sleep patterns, eating habits, body temperature and hormone levels are driven by the rhythmic activity of body's circadian clock. Travel across time zones or shift work can knock those rhythms out of whack, possibly leading to sleep problems, bipolar disorder, metabolic syndrome and even cancer. The lack of convenient and reliable methods to monitor the internal clock's activity has severely limited the study of circadian-related disease, but now, scientists report that they can easily track the circadian rhythms by analyzing a person's plucked hairs. The finding could one day help doctors diagnose and treat patients suffering from circadian dysfunction. The body's master clock, located in the brain region called the hypothalamus, is set by light, which activates clock genes that are responsible for keeping this timekeeper ticking correctly. Within the past decade, scientists have discovered that organs outside the brain (such as the skin, liver and pancreas) also keep track of time with 24-hour fluctuations in clock gene expression. Previous studies have attempted to monitor molecular timekeeping in blood cells or in cells lining the mouth, but these approaches are technically challenging. In an attempt to develop a simpler, noninvasive method to clock circadian rhythms, researchers led by Makoto Akashi of the Research Institute for Time Studies at Yamaguchi University in Japan obtained hairs plucked from volunteers' heads or chins and analyzed clock gene expression in hair follicle cells. They report online this week in the Proceedings of the National Academy of Sciences that the patterns of circadian gene expression in the hair follicle cells accurately reflected the subjects' behavioral rhythms, "demonstrating that this strategy is appropriate for evaluating the human peripheral circadian clock." © 2010 Scientific American

Dr. Shelby Freedman Harris and Dr. Michael Thorpy of Montefiore Medical Center respond: Delayed sleep phase disorder and insomnia are two different disorders. Patients with delayed sleep phase disorder, or D.S.P.D., usually have difficulty falling asleep, but once they fall asleep they have no difficulty obtaining a full night’s sleep and typically sleep until late morning or early afternoon. Those with insomnia, on the other hand, may have difficulty falling asleep or staying asleep, wake too early or feel that sleep is not restorative. Most insomnia patients have a combination of these symptoms, such as difficulty falling asleep as well as staying asleep. Insomnia is also usually due to some specific cause, such as a medical or psychiatric disorder like depression. Our bodies are biologically programmed, through circadian rhythms, to sleep at night and be alert during the day. In some people, these rhythms can shift, causing sleep and wake times to fall outside a desired schedule. In delayed sleep phase disorder, the sleep cycle is pushed later into the night, with a delayed natural morning wake time. Many patients with delayed sleep phase disorder consider themselves “night owls.” It’s common for us to delay our sleep and wake times because of late-night parties and other social activities, but this does not mean we have the disorder. People with delayed sleep phase disorder are unable to return to a normal schedule, despite trying, and end up spending a prolonged time in bed awake before falling asleep. If you are able to fall asleep easily on resuming a normal bedtime after a few late nights, then you do not have delayed sleep phase disorder. Copyright 2010 The New York Times Company

By Jordan Lite Despite kids’ protests, enforcing early bedtimes may be good for their mental health. Teens who are allowed to go to bed later are more likely to suffer from depression—probably for the simple reason that they are not getting enough sleep, a recent study suggests. Columbia University scientists found that depression was 24 percent more common in teens whose parents let them go to bed at midnight or later than in kids whose moms and dads required them to hit the pillow by 10 p.m. The night owls were also 20 percent more likely to have suicidal thoughts. Teens with bedtimes of midnight or later got an average of seven and a half hours of sleep, whereas those with a lights-out of 10 p.m. or earlier got an average of eight hours and 10 minutes. Although the association between later bedtimes and depression was greater before controlling for parents’ marital status and poverty level, it remained statistically significant after taking those things into account—as well as teens’ perceptions of how much their parents cared about them. The researchers looked at parent-enforced bedtimes—as opposed to simply logging hours slept—to rule out the possibility that depression was causing some kids to sleep less, rather than the other way around. Earlier work supports the idea that too little sleep may lead to depression. Research at the University of London showed that children who suffer from insomnia are at increased risk of developing depression in their tweens and teens. © 2010 Scientific American

By THE NEW YORK TIMES What happens when your work schedule interferes with a good night’s sleep and you find yourself growing increasingly exhausted? One reader asks the Consults blog. Q. I am a 51-year-old woman health care worker, and I’ve worked nights for about four years now. My problem is that I have not been able to sleep well during the day; I do not get more than two hours’ worth of sleep. I do not want to take a sleep aid because I do not want to become dependent on drugs. Is there any help for me without a sleep aid? Is this a sleep disorder? I notice I have been putting on weight since I started working nights. Charmaine, The Bronx A. Dr. Shelby Freedman Harris and Dr. Michael Thorpy of Montefiore Medical Center respond: Your question touches on a topic in sleep medicine that is increasingly gaining both research and clinical interest: the issue of shift work and insomnia. Shift work refers to anything outside of the traditional daytime work schedule. It mainly affects people who work nights but can also affect those who start work early, before 6:30 a.m., or in the evening, after 4:30 p.m. Our bodies are biologically programmed through circadian rhythms to sleep at night and be alert during the day. When we start to play around with these rhythms — by working during hours when our body physiologically desires sleep, for example — we are at risk of developing what’s known as shift work sleep disorder. Copyright 2010 The New York Times Company

Christopher Nolan’s Inception is a film about a time when we have the power to enter into each other’s dreams, and actively steer the dream’s course to implant an idea in the dreamer. The film raises the issue of how much we understand about the neuroscience of dreams. Due to its need for invasive experiments, neuroscience typically works with non-human animals, which raises a significant difficulty: how do you know that a rat is dreaming? You can’t wake it up from REM sleep and ask. (Well, you can, but don’t expect a cogent response.) There’s no accepted objective indicator that a person or animal is having a dream, as opposed to sleeping. But, we can still learn something useful by looking at the neuroscience of sleep. The neuroscience of sleep has told us a few important things over the years. For example, we know that our pattern of sleep and wakefulness (the “circadian rhythm”) has much of its basis in the activity of the suprachiasmatic nucleus, a rice-grained-sized group of cells just above where the optic nerves from our eyes crossover. We know that our free running rhythm—what we go to if we are completely in the dark, with no indicator of solar activity—is slightly over 24 hours, and that the length of the rhythm can be affected by things like cannabinoids found in pot. We know that the brain activity of a person dreaming is very similar to that of an awake person—were it not for the fact that our body is paralyzed during dreaming, we’d probably do a lot of things we’d regret. While we’ve made a lot of progress in understanding sleep, we’ve a long way to go to understand dreaming. What makes it a challenge, perhaps as big a challenge as understanding consciousness itself, is the subjective aspect of dreaming. For example, we know that vivid dreaming occurs during REM sleep in humans. We also know that other animals have REM sleep. Do they also dream?

by Gisela Telis Sound sleepers share a surprising secret: a bustling brain. A new study reports that people who can sleep through anything show more frequent bursts of brain activity called sleep spindles than do their light-sleeping counterparts. Researchers say the discovery could lead to spindle-enhancing techniques that offer lighter sleepers a chance at dead-to-the-world rest. Sleep spindles happen only during sleep, when brain waves slow. Scientists first spotted them in the 1930s, but they didn't suspect they were involved in how deeply people sleep. For decades, researchers instead chalked up the vast variability between light and heavy sleepers to differences in sleep stage; sound sleepers were thought to spend more of their repose in the deeper stages of sleep. Then in the 1990s, scientists tracked down the spindle's source: the thalamus, a brain region that regulates sleep and also processes and relays sensory information to the cerebral cortex. The spindle-thalamus link made it "logical that the sleep spindle would play a role in regulating sensory input while we sleep," says Jeffrey Ellenbogen, a sleep researcher at Harvard Medical School and Massachusetts General Hospital in Boston. "But no one had actually shown this." So Ellenbogen and colleagues invited 12 people to spend 3 nights in his lab's cushy digs. Presented with comfy beds and soundproof rooms, the subjects slept peacefully through the first night while the researchers measured their baseline brain waves. During the next 2 nights, the team played an assortment of 14 different sounds, including flushing toilets, loud conversations, ringing phones, and car traffic, 40 to 50 times throughout the night, gradually raising the volume of each sound until each sleeper stirred. © 2010 American Association for the Advancement of Science.

By Katherine Harmon Having a mixed up body clock has been linked to a vast array of ailments, including obesity and bipolar disorder. And researchers are still trying to understand just how these cyclical signals influence aspects of our cellular and organ system activity. Now, a study published online August 3 in Cell Metabolism shows that in mice, a disrupted circadian rhythm spurs an increase in triglycerides—heightened levels of which have been linked to heart disease and metabolic syndrome in humans. To find this link, researchers compared normal lab mice to those bred to have dysfunctional sleep-wake cycles. As nocturnal animals, the control mice had the lowest levels of triglycerides at night, when they were most active, and higher levels during the daytime rest period. The mice with out-of-whack cycles kept confused hours, fed longer and were less active overall. These mutant mice also had far less fluctuation in their triglyceride levels. "We show that the normal up and down [of triglycerides] is lost in clock mutants," M. Mahmood Hussain, of the Department of Cell Biology and Pediatrics at the State University of New York Downstate Medical Center in Brooklyn and coauthor of the paper, said in a prepared statement. The mutant mice had "high triglycerides all the time," he noted. © 2010 Scientific American,

by Karin Zeitvogel, AFP People who sleep more or fewer than seven hours a day, including naps, are increasing their risk for cardiovascular disease, the leading cause of death in the United States, a study published Sunday shows. Sleeping fewer than five hours a day, including naps, more than doubles the risk of being diagnosed with angina, coronary heart disease, heart attack or stroke, the study conducted by researchers at West Virginia University's (WVU) faculty of medicine and published in the journal Sleep says. And sleeping more than seven hours also increases the risk of cardiovascular disease, it says. Study participants who said they slept nine hours or longer a day were one-and-a-half times more likely than seven-hour sleepers to develop cardiovascular disease, the study found. The most at-risk group was adults under 60 years of age who slept five hours or fewer a night. They increased their risk of developing cardiovascular disease more than threefold compared to people who sleep seven hours. Women who skimped on sleep, getting five hours or fewer a day, including naps, were more than two-and-a-half times as likely to develop cardiovascular disease. Short sleep duration was associated with angina, while both sleeping too little and sleeping too much were associated with heart attack and stroke, the study says. Copyright © 2010 Discovery Communications, LLC.

by Nazlie Latefi People who smoke pot can feel lost in time—for some, it's part of the draw. Now researchers may have figured out one reason why cannabinoids, the psychoactive compounds in marijuana and hashish, make people feel this way; they disrupt the body's internal clock. Sleeping, eating, and other activities are all part of a 24-hour physiological cycle known as the circadian rhythm. This internal clock is controlled by neurons in a region of the brain called the suprachiasmatic nucleus (SCN). The SCN normally uses light to reset the clock. That's what happens when you travel from one time zone to another. But absent any sensory input, SCN neurons will still maintain a circadian rhythm: People and animals kept in total darkness continue to eat and sleep at the usual times. Several years ago, researchers discovered that SCN neurons possess receptors for cannabinoids. In the new study, a team led by Yale University circadian biologist Anthony van den Pol tried to figure out what role these receptors play. The researchers housed 42 mice in total darkness for 2 weeks to synchronize their internal clocks. In this environment the animals cycled through active and inactive phases lasting about 12 hours each. After 2 weeks, the researchers shined a light into some of the cages shortly after the mice had entered their active phase. Because mice are nocturnal, they became active about 2 hours later in the day than did mice not exposed to light, a phenomenon called "phase delay." But mice given brain injections of cannabinoids before light exposure exhibited much less of a phase delay; they became active only 1 hour later than did animals not exposed to light. © 2010 American Association for the Advancement of Science.

By Jennifer LaRue Huget A number of my apparently sleep-deprived friends and colleagues, upon learning I planned to write about foods that might help people sleep better, have told me they're eager to see what I come up with. I so hate to disappoint them. But it turns out science has yet to find a magical food that can send us right to slumberland. "The bad news for people trying to talk about food and sleep is that . . . generally it's hard to find foods that help with sleep," says Michael Grandner, a sleep researcher at the University of Pennsylvania's Center for Sleep and Respiratory Neurobiology. "The easier question," Grandner says, "is what are the things to avoid?" Though you might expect caffeine to top that list, Grandner's most recent research, published February in the journal Sleep Medicine, found otherwise. Tracking the diets and sleep habits of 459 women enrolled in the federal government's 15-year Women's Health Initiative, he found that fat was the main nutrient (out of dozens tracked) associated with getting less sleep. "The more fat you ate, the less you slept," he says. Women who ate the most fat slept for shorter times and took more naps, a sign that they didn't get enough restful sleep at night. (He believes his findings apply to the broader population, not just older women.) © 2010 The Washington Post Company

By SARAH KERSHAW ALBUQUERQUE — Her car is racing at a terrifying speed through the streets of a large city, and something gruesome, something with giant eyeballs, is chasing her, closing in fast. It was a dream, of course, and after Emily Gurule, a 50-year-old high school teacher, related it to Dr. Barry Krakow, he did not ask her to unpack its symbolism. He simply told her to think of a new one. “In your mind, with thinking and picturing, take a few minutes, close your eyes, and I want you to change the dream any way you wish,” said Dr. Krakow, founder of the P.T.S.D. Sleep Clinic at the Maimonides Sleep Arts and Sciences center here and a leading researcher of nightmares. And so the black car became a white Cadillac, traveling at a gentle speed with nothing chasing it. The eyeballs became bubbles, floating serenely above the city. “We call that a new dream,” Dr. Krakow told Ms. Gurule. “The bad dream is over there” — he pointed across the room — “and we’re not dealing with that. We’re dealing with the new dream.” The technique, used while patients are awake, is called scripting or dream mastery and is part of imagery rehearsal therapy, which Dr. Krakow helped develop. The therapy is being used to treat a growing number of nightmare sufferers. In recent years, nightmares have increasingly been viewed as a distinct disorder, and researchers have produced a growing body of empirical evidence that this kind of cognitive therapy can help reduce their frequency and intensity, or even eliminate them. Copyright 2010 The New York Times Company

by Sujata Gupta WHAT happens when you take blind mice and see how they run? It turns out they can identify objects using receptors in the eye that were previously thought to have no role in forming images. Since humans possess the same receptors, the finding could point the way to giving blind people some ability to see. Mice, and humans, have three types of light-detecting receptor in the eye. Rods and cones detect light, darkness, shape and colour, and make normal sight possible. Receptors of the third type, the melanopsin-containing ganglion cells (MCGCs), were until now thought only to respond to light over longer periods of time, to help moderate patterns of sleep and wakefulness. To investigate their role in vision, Samer Hattar of the Krieger School of Arts and Sciences at Johns Hopkins University in Baltimore, Maryland, and colleagues engineered mice to lack rods and cones. When these mice were placed in a maze, they were able to identify a lever with a visible pattern on it which allowed them to escape. Mice that lacked rods, cones and MCGCs could not find the lever. In another task, the team found that the MCGC mice could follow the movement of a rotating drum (Neuron, DOI: 10.1016/j.neuron.2010.05.023). This suggests MCGCs can form "low-acuity yet measurable images", Hatter says. Tom Cronin at the University of Maryland notes that the mice in the experiment behave like people with "blindsight", who can navigate round objects without consciously perceiving them. "It's mind-boggling but I suspect that the mice are doing something like that," he says. © Copyright Reed Business Information Ltd.

By THE NEW YORK TIMES Q. What are the risks of long-term use of prescription drugs to combat insomnia?Claudia, New York, N.Y. A. Dr. Thorpy and Dr. Harris respond: In our sleep center, we use several types of prescription medications to treat insomnia. The most well known are drugs like Ambien (zolpidem), Lunesta (escopiclone) and Sonata (zaleplon) — sometimes referred to as the “Z-drugs” or hypnotics. They all affect a brain structure called the GABA receptor, which is widely found throughout the brain and has many functions; their main effect is to dampen arousal, thereby allowing sleep to occur. There are also non-GABA sleep medications like Rozerem (ramelteon), which reduces arousal by affecting the receptor for the hormone melatonin. Before using any of these medications, it is vital to understand the underlying cause of the insomnia, since other treatments may be more appropriate. If depression is the cause of poor sleep, for example, antidepressants or lifestyle changes may be the most appropriate course. Insomnia has now been shown to be associated with a range of underlying medical and psychological disorders, and it is therefore an important condition to treat. We usually use both behavioral and medication therapy, either alone or together, to get the best response. The aim is to have the patient come off the sleep medication completely when the insomnia resolves. Some people, though, have chronic insomnia that is associated with a medical or psychiatric disorder and need to continue medication very long-term. They can do so without any untoward effects. Copyright 2010 The New York Times Company

By Lindsey Tanner Want happier, more alert teenagers? Let them sleep in a little. A new study reveals that delaying the school day by 30 minutes results in teens who are less sleepy and depressed. Scientists say that teens tend to be in their deepest sleep around dawn, when they typically need to arise for school. Interrupting that sleep can leave them groggy. Giving teens 30 extra minutes to start their school day leads to more alertness in class, better moods, less tardiness, and even healthier breakfasts, a small study found. "The results were stunning. There's no other word to use," said Patricia Moss, academic dean at the Rhode Island boarding school where the study was done. "We didn't think we'd get that much bang for the buck." The results appear in July's Archives of Pediatrics & Adolescent Medicine. The results mirror those at a few schools that have delayed starting times more than half an hour. Researchers say there's a reason why even 30 minutes can make a big difference. Teens tend to be in their deepest sleep around dawn -- when they typically need to arise for school. Interrupting that sleep can leave them groggy, especially since they also tend to have trouble falling asleep before 11 p.m. © 2010 Associated Press/AP Online

By Christine Soares A restful night’s sleep might make a cup of coffee less of a desperate need first thing in the morning. But pharmaceutical companies are looking into whether the latest pills to promise sound, natural sleep could also play an active role in overcoming even the most powerful addictions. The new sleep aids block the activity of brain peptides called orexins. These tiny proteins keep us wide awake and attentive during the day, and they also govern some stimulating effects of addictive drugs. Orexins do not cause addiction or relapse directly, but neither happens without the peptides’ participation. The intriguing connection between sleep and addiction has long been observed in people who suffer from narcolepsy—a disorder that causes sudden-onset sleep. Although narcoleptics were sometimes treated with potent amphetamines to help them stay awake, they never became addicted to the drugs. By 1998 genetic detective work had traced the cause of narcolepsy to mutations in the genes for orexins or their receptors—discoveries that revealed both the peptides’ existence and their critical role in keeping the brain awake. Efforts to turn those insights into novel insomnia treatments have led to several compounds that are now in late-stage clinical trials. The same companies developing these sleep aids are also investigat-ing orexins’ role in addiction through research in animals. In a recent study Davide Quarta and his co-workers at GlaxoSmithKline Medicines Research Center in Verona, Italy, confirmed that when the company’s experimental orexin blocker SB-334867 was administered to rats along with amphetamine their brains released less dopamine and they became less sensitized to the stimulant than controls did, even with repeated doses. Sensitized neurons grow extra receptors for the craved drug, demanding more of it to achieve stimulation, thereby fueling a cycle that leads to addiction. © 2010 Scientific American,

Beds that cost up to $60,000 each are now available, note Dr. Michael Thorpy and Shelby Freedman Harris of the Sleep-Wake Disorders Center at Montefiore Medical Center in the Bronx. But is a better mattress the secret to curing insomnia? Drs. Thorpy and Harris recently responded to questions about insomnia on the Consults blog. Here, they address which type of mattress is best for a sound night’s sleep and whether light or noise might be reasons for sleeping poorly. Q. Is there any research that shows if the kind of mattress you have affects sleep? Dee, Western New York A. It is common for people with insomnia to wonder if their bed, or some other environmental factor like light or noise, is the reason for their sleeping poorly. Sometimes an uncomfortable mattress is the cause of the sleep disturbance, but most often it is not. Very few studies have analyzed how the type of mattress affects sleep quality, and they’ve generally involved a small number of healthy subjects or patients who are in pain. The results have been variable, with some preferring a soft surface and others preferring a hard surface. No clear benefit of any mattress type has been shown. People in some jungle cultures, or even hikers or campers, who sleep on mats on the hard ground can usually get a good night’s sleep if they do it often enough. It is largely a matter of conditioning to the environment that allows a person to sleep comfortably. Problems can arise, however, with sudden changes, like staying in a hotel overnight, when a new bed or environment can be a factor in disrupting sleep. Copyright 2010 The New York Times Company

By Carolyn Butler No matter what I do, and despite the fact that my baby has arisen at the crack of dawn for well over a year now, I just can't seem to turn myself into a morning person. My body simply refuses to shut down much before midnight, and so I work, pay bills and watch terrible reality-TV reruns until the wee hours, only to be dog-tired and disagreeable come 6 a.m., when my live little alarm clock begins wailing for me. Even when I force myself to go to bed on the early side or when my husband lets me sleep in on a Saturday, waking up always seems a chore. My brother-in-law, on the other hand, is known for unabashedly yawning in people's faces starting right around 8:30 at night, whether he's at home, a family dinner or the theater. I ran into him bright and early the other morning on my way to Starbucks, when I was so beat that I could barely communicate -- and he was clear-eyed and chipper, heading off on a long run. What makes one person greet the day with smiles and energy, and another hide under the covers until the last possible moment? It's a combination of genetics, the environment and our lifestyle choices, says sleep specialist Mark Wu, an assistant professor in neurology at Johns Hopkins Medicine. He explains that your body's natural circadian rhythms, which cycle up and down over an average 24.1 hours, control sleep and wakefulness and differ from person to person. How much sleep you've had lately also makes a difference, influencing how great your body's drive for more shut-eye is. © 2010 The Washington Post Company

Chapter 14. Biological Rhythms, Sleep, and Dreaming