What Happens when We Sleep?
The Relationship between Biological Rhythms and Sleep
Biological rhythms are periodic physiological fluctuations, such as the rise and fall of hormones, and the acceleration and deceleration of brain activity. Biological rhythms are controlled by biological clocks: The annual and seasonal cycles govern the migration, hibernation and eating habits of certain animals; the twenty-eight-day cycle governs the menstrual cycle in women; and the twenty-four-hour cycle, also known as the Circadian Rhythm, from the Latin words circa ("about" or "circular") and dies ("day"), governs the stages of sleep and wakefulness, and daily changes of body temperature, blood pressure and blood sugar levels. In a day, our body temperature fluctuates by 3 degrees Fahrenheit, peaks in the afternoon, and reaches its lowest point from 2am to 5am.
The circadian rhythm is controlled primarily by the Suprachiasmatic Nucleus (SCN), located inside the front portion of the hypothalamus. It regulates daily physiological fluctuations based on changes in visual information, particularly changes on the different properties of light, as recorded by the retina. Although the SCN rely mainly on sunlight to regulate the circadian rhythm, the body is still capable of completing the twenty-four-hour cycle even without sunlight. In an experiment by French scientist Michael Siffre, where he stayed at a midnight cave near Del Rio, Texas, for 6 months, starting February 6, 1972, he found that circadian cycle is equivalent to 25 hours on average. Recent replications of this study, however, showed that circadian rhythm lasts closer to 24 hours on average.
The SCN governs the circadian rhythm by regulating the release of certain hormones in the hypothalamus and reticular formation. The hypothalamus is the part of the brain involved in monitoring body temperature and hunger, while reticular formation is primarily responsible for monitoring stages of sleep and wakefulness. This is why damage to the reticular formation may cause instant death or comatose.
The circadian rhythm may be desynchronized, as with jet lags and rotating work shifts. The effects of jet lag can be pervasive, because it desynchronizes two or more aspects of the circadian rhythm. For example, jet lags distort the body's conditioned response to sleep when body temperature lowers and when the SCN releases large doses of cortisol hormone. Desynchronization of the circadian rhythm due to jet lags may be treated by taking artificial melatonin to induce sleep and advance the biological clock. The problem, however, with taking artificial melatonin is that side effects are unknown and not fully documented. Rotating work shifts can also affect and desynchronize the circadian rhythm. In 1979, a nuclear accident at Three Mile Island forced its employees to work in constant rotating shifts for 6 consecutive weeks. It was documented that employees who work in this condition slept mostly at work, were less productive, and developed increased risk for heart diseases and gastrointestinal problems. The effects on workers aged more than 50 years old are more adverse, because this age group requires more than 9 hours of daily sleep and are morning types. Negative effects of rotating work shifts may be controlled by increasing bright light at work, sleeping at total darkness at home, and sleeping right before and after work.
The Benefits of Sleep and Effects of Sleep Deprivation
Sleep benefits us in many ways:
- Sleep has growth and restorative benefits. According to the National Institute for Neurological Disorders and Stroke (2001), growth hormones are released in children during deep sleep. It is also during sleep that the body increases protein production, while breakdown of protein also decreases, both processes that are important for cell growth and repair of damaged cells. Neuroscientists also stress that sleep reduces the harmful effects of too much by-products of cellular activities during wake period.
- Sleep has cognitive benefits. Roehrs & Roth (1998) found that alertness increased even for well-rested 8-hour sleepers upon sleeping for 2 more hours every day. Research also shows that sleep has important role in the storage and maintenance of memory. It was found out that REM sleep, a stage of sleep (will be discussed below), is linked with the formation of memories. Stickgold and Hobson (2000) also found that students who studied all night for an exam significantly scored lower than students who had a good night sleep. Because there is less sensory input during sleep, more time is allotted for memory integration.
- Sleep has adaptive benefits. Evolutionary psychologists believe that sleep is related to day-hunting in humans. Day-hunting is thought to be safer, with less possible accidents, for humans. Also, they found that predators tend to sleep more than preys. On average, horses sleep for 2.9 hours every day, sheep and cows sleep for 3.8 hours, humans sleep for 8 hours, rabbits sleep for 8.4 hours, rhesus monkeys sleep for 9.6 hours, foxes sleep for 9.8 hours, bats sleep for 9.9 hours, cats sleep for 14.5 hours, armadillos sleep for 18.5 hours.
James Maas (1998) said that humans need more than 8 hours of daily sleep to achieve optimal functioning; however, according to a national survey, with 1,000 American adults, of the National Sleep Foundation (2000), 63% sleep less than 8 hours every day, where 31% of those sleep even less than 7 hours every day. All said that they try to catch up with their sleep, but all still sleep less than 8 hours on weekends.
To illustrate the negative effects of sleep deprivation, take the case of Randi Gardner. Randi Gardner is the record-holder of the longest observed period of total sleep deprivation. He stayed awake for 11 days (264 hours, to be exact). He did this for a high school science project when he was 17 years old. He experienced hallucinations and motor problems, but consistently won over William Dement, a sleep researcher, in an arcade game. Perhaps the national TV coverage he received during the course of the experiment helped in keeping him awake. He claimed full recovery after sleeping for 14 hours and 40 minutes.
Besides hallucinations and motor problems experienced by Gardner, research shows that the frequency of brain activity in the thalamus and prefrontal cortex decreased with sleep deprivation (Thomas & others, 2001); that the complexity of brain activity observed by EEG decreased during total sleep deprivation (Jung & others, 2001); that attention span lessens with sleep deprivation (Doran, van Dongen & Dinges, 2001); and that certain aspects of decision-making, such as dealing with the unexpected, undertaking innovations, revising plans, and effective communication are affected negatively with sleep deprivation (Harrison & Horne, 2000). Furthermore, 40% Americans said that sleepiness affects their work for a few days per month, and that 22% are affected per week.
Stages of Sleep and Wakefulness
The stages of sleep and wakefulness are derived from observed EEG patterns of brain activity while awake and while asleep.
Wakefulness is characterized with two waves - alpha waves and beta waves. The first stage of wakefulness is composed of beta waves, and is aptly called the Beta-Wave Stage. Beta waves have the highest frequency and the lowest amplitude, and are desynchronous due to variations of sensory information we receive when we are awake. The second stage of wakefulness is composed of alpha waves, and is aptly called the Alpha-Wave Stage. Alpha waves have lower frequency and higher amplitude than beta waves, but they are more synchronous, reflecting relaxation and drowsiness.
Sleep, on the other hand, is characterized with three waves - theta waves, delta waves, and a series of waves similar to alpha waves. There are 5 stages of sleep. The first stage of sleep is composed of normal theta waves, while the second stage of sleep is composed of theta waves with occasional spindles. Theta waves have lower frequency and higher amplitude than alpha waves. The first two stages of sleep are light, so that most that are awakened at this stage report not being able to sleep. The third and fourth stages of sleep are composed of both theta waves and delta waves. Delta waves have the lowest frequency and the highest amplitude. Delta waves occur less than 50% of the time in the third stage, and more than 50% of the time in the fourth stage. The fourth stage of sleep is the deepest stage, where sleepers are most difficult to wake, and if awakened, report feeling confused and distorted. Lastly, the fifth stage of sleep is composed of a series of waves similar to alpha waves. It is also called the Rapid-Eye-Movement (REM) stage because eyes are observed to move as if watching a scene. Dreams occur in other stages, but it is in the fifth stage of sleep that dreams occur the most. Even those who claim that they rarely dream report having dreams when awakened at the fifth stage of sleep. Based on reports of research participants awakened at the fifth stage of sleep, dreams are longer, more vivid, motorically animated, more emotionally charged, more fantastical, and less related to events preceding sleep. According to Empson and Clarke (1970), the REM stage has important role in memory formation. They found that participants who previously memorized phrases significantly recalled less when awakened before than after the REM stage.
The stages of sleep occur in periodic cycles. The first and second stages of sleep, combined, always last 60% of the time. The third and fourth stages of sleep, combined, last 20% of the time, and decreases on subsequent cycles as the night progresses. The fifth stage of sleep also lasts 20% of the time, and increases on subsequent cycles as the night progresses.
Patterns of Sleep across the Ages
Humans sleep less and dream less as they age. On average, 1-15 day-old babies sleep for 16 hours and dream for 8 hours in a day; 6-24 month-old babies sleep for 13-14 hours and dream for 6-4 hours; 10 year-olds sleep for 10 hours and dream for 2 hours; 20-90 year-olds sleep for 8-6 hours and dream for 2-1.5 hours.
Adolescents sleep for approximately 9 hours and 25 minutes if given the opportunity to sleep as long as they like. Older adolescents, aged 16-18 years, sleep and wake up later than younger adolescents, aged 13-15 years, due to delayed nightly release of melatonin in older adolescents, from 9:30 pm to 10:30 pm. Edina (1997) asked a few Minnesota schools to delay the start of classes for older adolescents from 7:15 am to 8:40 am. As observed by the school's administration, the change brought lesser discipline problems, less illnesses, and better test scores for high school students.
Unlike older adolescents, middle-age adults experience early nightly release of melatonin, and are consequently off to bed early. They are less likely to sleep through the entire night, and spend less time in the deep sleep stage than their younger counterparts. Old adults, however, experience some degree of insomnia.
Types of Sleep Disorders
40 million and 20 million Americans experience chronic and occasional sleep disorders, respectively, in a year. The common types of sleep disorder are:
- Insomnia. Insomnia is a type of sleep disorder characterized with the inability to sleep. It involves difficulty falling asleep, waking up during the night, and waking up too early. It affects 20% of all Americans and is most common among women, old adults and those who are thin, depressed and stressed. Mild insomnia is often treated behaviorally, that is, by developing good sleeping habits. Edinger et al. (2001) observed that those inflicted with mild insomnia sleep better at night when they spend longer time awake in the day. They suggest that those inflicted with mild insomnia should avoid taking naps on daytime and to start using alarm clocks to force themselves out of bed. Sleeping pills, on the other hand, are prescribed for those with short-term insomnia. This is because sleeping pills are immediately effective, but have reduced effects and can even interfere with attaining good night sleep if used long-term. Treatment for serious cases of insomnia, such as light therapy and artificial melatonin supplements, involve altering the circadian rhythm.
- Somnambulism. Somnambulism, a type of sleep disorder commonly referred to as sleep walking, actually has no relation with dreaming. It occurs during deep sleep, just before the REM stage, and thus happen mostly early in the night. Although difficult to do, somnambulists should be awakened, as they may harm themselves.
- Sleep Talking. Sleep talkers surprisingly respond coherently, but the accuracy of their response has not yet been investigated in research.
- Nightmares. Nightmares are frightening dreams that thematically involve danger. Those who experience nightmares often wake during the REM stage. Research shows that nightmares are positively correlated with stress. Nightmares peak at 3-6 years of age; and adolescents and young adults typically report having 4-8 nightmares in a year.
- Night Terror. Night terror involves experiencing sudden arousal and intense fear during sleep. Accompanying physiological reactions include rapid heart rate, fast-paced breathing, loud screams, heavy perspiration and troubled movements. Night terrors are less common than nightmares, and peak at 5-7 years of age.
- Narcolepsy. Narcolepsy is a type of sleep disorder characterized with an overpowering urge to sleep. Narcoleptics often fall asleep while talking or standing, and directly go to the REM stage without entering the other stages of sleep. Narcolepsy is suspected to be inherited, though sufficient proof has not yet been established. Treatment for narcolepsy often involves counseling to discover potential psychological causes.
- Sleep Apnea. Sleep apnea, or snoring, is a type of sleep disorder characterized with sudden stoppage of breathing. The windpipe fails to open due to failure of brain processes involved in respiration. Those inflicted with sleep apnea experience numerous brief awakenings at night to jumpstart the windpipe, although most are not aware of them. This leads to sleepiness during the day. Sleep apnea can cause high blood pressure, stroke and impotence. It affects 12 million Americans, and is common among infants and obese old adults.
Links between Sleep and Diseases
Certain physical and mental disorders affect sleep. Strokes and asthma attacks often occur during the night and early morning. Cytokines released by the body to fight infections are sleep-inducing, and a lot of medications are too. Depressed individuals are often insomniacs, complaining that they wake up too early and can't get back to sleep. Depressed individuals also spend less time in deep sleep than those who are not depressed.