The Brain

by Gary L Wenk

  Why is sleep so important?

  What is so critical about sleep that your brain can justify shutting down consciousness? When brains were evolving, sleeping was a very dangerous thing to do. Being unconscious of your surroundings places you in danger of being discovered unaware and vulnerable to attack. Clearly, something about turning off your conscious awareness of the environment is critical to your survival, otherwise such risky behavior as falling asleep would not be permitted.

  One recent theory states that with prolonged wakefulness the waste products produced by normal brain function lead to functional deterioration. According to this theory, sleep is required for the removal of these waste products of brain metabolism from the interstitial space among brain cells where they accumulate. In addition to providing an opportunity for the brain to flush itself of debris, sleep also is associated with many different changes in wiring between neurons so that the communication between these neurons is improved or enhanced. This change in the communication between neurons underlies the formation of new memories. Essentially, while you are sleeping your brain is reactivating patterns of neural activity that it experienced during the previous day while you were awake. The brain appears to rewind the videotape of the day’s events, replay them, and delete memories that are weak, and possibly unimportant to you, and enhance memories that are strong and likely quite important to your survival. Essentially, sleeping and dreaming are ways that the brain has evolved to get rid of the chemical and mental debris that it collects during the day.

  You do not “fall” asleep. Sleep is not a passive process. Transitioning from being awake to being asleep is an active process that involves the activation of many different neurotransmitter systems. The most critical neurotransmitter for the transition into sleep is gamma-aminobutyric acid (GABA), the brain’s principal inhibitory neurotransmitter molecule. The amino acid neurotransmitter GABA always turns other neurons off. Drugs that enhance the action of GABA provide therapeutic benefits for a wide range of disorders, particularly for the treatment of anxiety and insomnia. For example, alcohol and many antianxiety drugs reduce brain activity by enhancing the actions of GABA at its protein receptors.

  What other daily rhythms do I experience?

  Each cell and organ system in your body follows a daily rhythm that is initiated by the rising sun every morning. One of the most important rhythms is body temperature. You are a warm-blooded animal, and your brain carefully defends you from cooling down to match the temperature of your environment. Your body temperature fluctuates rhythmically throughout the day. Every day you wake up cold and quickly become warmer, reaching a plateau in the morning that you maintain throughout the day. At the end of the day, you start cooling down again as you fall asleep. This is why it is best to keep the bedroom as cool as possible; at the end of the day, it assists with your transition into normal sleep.

  While you are sleeping, the nighttime is full of important rhythms, too. The onset of sleep is associated with many significant fluctuations in various hormones. When you fall asleep, your body releases growth hormone and other wound-healing chemicals; after all, the brain has learned that for the next few hours it has a chance to heal. Your body’s production of cholesterol and triglycerides increases just after you fall asleep, which explains why statin cholesterol-lowering drugs are most effective if taken at bedtime. Blood levels of the stress hormone cortisol are surprisingly high when you wake up in the morning and then gradually fall throughout the day. The rhythmic clocks in your brain also influence how you feel, how you think, and how drugs affect your brain. You perform mental tasks much better in the late morning and you are more vulnerable to the disruptive and toxic effects of alcohol and anesthetic drugs in the late evening.

  What happens when I disrupt these rhythms?

  Traveling across time zones is disruptive to brain and body performance, especially when flying in the same direction as the earth is spinning, namely, east, because then you experience the sun rising sooner than your brain expects. For example, visiting teams who fly east win an average of 37% of their games; visiting teams who fly west win an average of 44% of their games; obviously, it is best to be the home team. Scientists have learned that we need to maintain normal rhythms of waking each day and falling asleep each night at the same time; if we do not, the consequences can be far more negative than just losing to the home team.

  To study the impact on your brain and body of losing these normal rhythms, watch what happens when you isolate yourself from the influence of morning sunlight, for example, by living in an underground cave. At first, your major rhythms are maintained; you go to sleep at roughly the same time, become colder as the night progresses, and awaken at the usual times. Then, as you spend more time underground, without noticing it you start going to bed about one hour later every night. Why? Because, like most people, you do not have a true 24-hour clock in your brain. In fact, most of us are born with a 25-hour clock in our brains. (Some people may even have 36-hour clocks!) This is why everyone finds it so easy to stay awake one extra hour to watch a favorite television program and why it is so very hard to go to sleep earlier than usual.

  In addition to falling asleep one hour later each day, because you do not have access to the morning sunshine, your body begins to make other adjustments. One major change involves the rhythmic fluctuation in your body temperature, particularly the point at which you are the coldest each day. Before moving into the cave you would have experienced your daily minimum temperature early in the morning, just before waking. Now that you are living in a cave you start feeling colder at mid-day, and then at the beginning of the day. These alterations in essential biorhythms indicate that your body is undergoing some very significant disruptions. Unfortunately, these disruptions in your biorhythms have lingering effects after you emerge from your underground home. One of the most common negative consequences is depression. Current thinking is that people who are vulnerable to depression might have their illness triggered by disruption of their healthy sleep-wake pattern. Alternatively, recent studies of the genes involved in controlling the brain’s circadian clock mechanism suggest that inherited abnormalities in these genes may underlie mood disorders such as bipolar illness. This is why it is so important for people with major depressive disorder, or bipolar disorder, to maintain an inflexible sleep–wake pattern; that is, these patients must go to bed at the same time every night. Disruption of the normal sleep–wake pattern also might underlie why traveling across multiple time zones can induce a depressive episode in vulnerable people.

  How are eating and sleeping related?

  What you eat before bedtime also might improve your chances of getting a good night’s sleep. A recent study suggests that eating something sweet might help induce drowsiness. Elevated blood sugar levels have been shown to increase the activity of neurons that promote sleep. These neurons live in a region of the brain that lacks a blood–brain barrier; thus, when they sense the presence of sugar in the blood, they make you feel drowsy. This might explain why we feel like taking a nap after eating a large meal. This is just one more bit of evidence demonstrating your brain’s significant requirement for sugar in order to maintain normal function.

  During the day, fat is deposited, muscles increase their metabolism of fats and sugars, your liver is busy producing glycogen (a form of sugar storage) and bile (in order to absorb fats), while your pancreas is busy releasing insulin in response to eating. At night, these processes reverse; for example, fat catabolism is increased. This explains why getting adequate amounts of sleep, that is, seven to eight hours, is so important for the maintenance of a normal body weight. While you were sleeping and dreaming, your brain and body used up a lot of energy; thus, as soon as you awaken, your body needs to replenish itself.

  How does your brain control its rhythms?

  The sun has arisen and you are awake; what are you going to do now? Eat.

  These two events have occurred together for as long as your brain has been
evolving on this planet. Thus, it is not at all surprising that your brain uses the same neurotransmitter molecule, called orexin, to both arouse you from sleep and then induce you to eat. There are only about 70,000 orexin neurons in the entire human brain; however, these relatively few neurons control many other better known neurotransmitter systems, such as serotonin, histamine, and acetylcholine, whose roles are to control your level of arousal and attention throughout the day and night. You should view orexin as the one ring to control them all.

  What would happen if your orexin neurons started dying? The answer is that you would develop narcolepsy, an incurable neurological disorder. Narcolepsy is an autoimmune disorder, which means that your body is attacking a part of itself; in this case, the orexin neurons in your brain are being attacked by your body’s immune system. The symptoms of narcolepsy are excessive daytime sleepiness, sleep paralysis, hypnagogic hallucinations (that occur at the onset of sleep) and disturbed nocturnal sleep. People with narcolepsy experience intermittent, uncontrollable episodes of falling asleep during the daytime; these symptoms are sometimes evoked by strong emotions. People with narcolepsy begin dreaming almost immediately upon losing consciousness. This is unusual because typically most people do not start dreaming for almost 120 minutes after falling asleep. Recent studies suggest that the loss of orexin neurons, long-term stress, and sleep deprivation may be associated with obesity and age-related decline in cognitive abilities. This connection will become more obvious as the details of sleep are discussed below.

  Sleep is composed of two distinct phases: non-rapid eye movement (non-REM) sleep and rapid eye movement (REM) sleep. Non-REM sleep has four unique stages, ranging from light to deep sleep. After initially falling asleep, you spend about 90 minutes sequentially passing through deeper and deeper stages of non-REM sleep; then you reverse this process and slip into lighter states of non-REM sleep before finally switching over to your first REM sleep episode. REM sleep is the most mentally active stage of sleep and is when dreaming most often occurs. During REM, or dream sleep, your eyes move back and forth beneath the eyelids; this is why the term rapid eye movement sleep was given to this stage of sleeping. Your brain experiences about six to eight non-REM-to-REM cycles of sleep during a typical night of sleep. Normal amounts of REM sleep facilitate creative problem solving. Thus, nighttime dreaming is a necessary component of making us smarter during the day. It is not, however, the total quantity of sleep that you have each night that is so critical; neuroscientists now understand that the quality of sleep each night (i.e., the balance between non-REM and REM sleep phases) underlies optimal brain function during the daytime.

  Why do I sometimes wake up paralyzed?

  During REM sleep, your muscles are actively paralyzed so that you do not act out your dreams and risk discovery by predators while you are vulnerable. During dreaming, the muscles of your body are inactivated by a complex interaction of many different neurotransmitter systems descending into the spinal cord. Sometimes as you transition into, or out of, your dream state, this paralysis can turn on too late or not turn off in time. This is called “REM atonia” and is experienced by about 60% of the population at least once in their lives. Waking up paralyzed is likely due to the failure of GABA-containing neurons in the brain to turn off. In a typical event, people report that they are fully awake but unable to move. Naturally, this causes considerable fear. Many people also report that they feel as though they are not alone in their bedroom when this happens—which adds to their fear. Typically, the paralysis disappears quickly without any lingering problems. Frequent wakening paralysis may indicate underlying problems with brainstem function; however, too little information is available at the current time to understand fully the reasons behind wakening paralysis.

  Why do some people act out their dreams?

  Some sleepers, usually elderly men, make abnormal flailing movements during sleep while acting out their dreams. These movements are potentially dangerous to their bed partners, who often suffer injuries inflicted by their larger male mates. This syndrome is call REM Behavior Disorder and is likely due to the degeneration of a group of GABA-containing neurons in the brainstem. Middle-aged men and women who demonstrate REM Behavior Disorder are at increased risk (by about 50%) of developing Parkinson’s disease about six years after this symptom appears. No particular lifestyle or nutritional behaviors, such as drinking caffeinated beverages, smoking, or consuming alcohol, altered the onset of symptoms. Recent evidence suggests that the symptoms of REM Behavior Disorder may occur quite early in life, possibly even during the second decade of life.

  What happens when I am dreaming?

  Everyone dreams about something. Even if you do not remember your dreams from last night, you did dream. It was once believed that you only dreamed during REM sleep. Now scientists realize that not all dreaming occurs during REM sleep; you also dream during non-REM sleep, although the nature of the dreams in each stage is quite different. What is your brain doing when you are dreaming? This is what we know from studying animals and humans when they are dreaming: During the day, your hippocampus was busy gathering sensory information associated with the events of your life. At night while you are in non-REM sleep, the hippocampus “shows movies” of the day’s events to the frontal cortex. These “movies” are presented in very compressed packets of neural information. The “movies” are presented about 11 times faster than the events actually occurred in real life. Why? Just consider the challenge your brain faces every night—lots of things happened during the previous 18 hours and all of this information needs to be processed within the much shorter period of time that you spend in REM sleep. Thus, because you spend much more time awake gathering information and knowledge than you spend asleep processing this information, your brain needs to work very hard while you are dreaming. In spite of this speedy processing by our neurons, most dreams have a realistic sense of time (more on this later). Dreams that occur during non-REM sleep tend to be simpler and have less narrative quality as compared with the more complex, and usually more emotional, dreams that occur during REM sleep.

  Dreams are about the people you know, places you have visited, places you have seen on television, or in movies, events that occurred in the books you have read, creations of your imagination, and the everyday normal events of your life. These thoughts and memories are the alphabet that writes the storyline of your dreams. For example, I am fairly certain that no one reading this page has ever dreamed of meeting me (unless, of course, you have met me). Furthermore, before the appearance of the Star Wars movies, no one ever dreamed of a Wookie or a Tatooine Canyon Krayt dragon. No one has ever dreamed of meeting an actual alien from another planet simply because no one on this planet has ever met one.

  The content of men’s dreams differs in important ways from that of women’s dreams. When hundreds of test subjects were awakened while dreaming, the women frequently reported seeing bright colors while the men reported seeing very few bright colors; most objects for men appear in washed-out pastels or shades of gray. Women often report seeing brighter colors during menstruation. Women also reported knowing the identity of the sexual partners they dreamed about and noticing their partner’s hands and face. In contrast, the men usually did not know the identity of their sexual partners and did not report seeing their partner’s face. Pay close attention to your own dreams, when you remember them, and notice how your dreams compared with these reports.

  Most dreams tend to emphasize strong emotions at the expense of reason. Neuroscientists believe that the presence of strong emotions while dreaming is related to the fact that a specific brain structure, the amygdala, is quite active during dreaming. What you fear while dreaming is likely also what you fear while awake. Your personality does not change when you are dreaming. Dream content reflects your basic waking conceptions; thus, if you are a devout Christian or Republican when you are awake, you will maintain similar views while dreaming.

  Why do I sometimes dream that I am being
buried alive?

  Why do some dreams involve the terrifying feeling of being buried alive or the feeling that it is difficult to breath? These dreams of suffocation usually occur during non-REM sleep when your respiration and heart rate are slowed down significantly. If you are dreaming while experiencing these physiological conditions, your brain incorporates their sensory qualities into your dream narrative. Sometimes, just being wrapped up in your bed sheets provides a sufficient sensory stimulus to induce a dream of suffocation.

  Are children’s dreams different?

  The dreams of children have not been studied as extensively as those of adults. In general, the dreams of children differ from those of adults and greatly depend upon the brain’s level of maturation. Children usually are first able to talk about their dream experience around the age of two years. Only 20% of the time do children under eight years of age awaken from REM sleep recalling dreams (as compared with 90% of adults). Some studies suggest that the content of young children’s dreams is often static and bland, not animated or emotional in the way that most adults experience dreams. Too often, however, these studies are confounded by the fact that the children were not fully aroused when questioned about their dreams. The dreams of a child do not regularly include the dreamer as an active character until after six or seven years of age, although some children report being an active participant as early as three years of age. This suggests that the appearance of dreaming, similar to many other higher cognitive abilities in young children, is due to a gradual, and highly variable, developmental process. The dreams of young adolescents, particularly in the case of recurrent dreams that tend to be more unpleasant, frequently involved confrontations with monsters or animals, followed by physical aggression, falling, and being chased.