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Consciousness is the awareness we have of ourselves and our environment. Different states of consciousness are associated with different patterns of brain waves. Brain waves are tracings of electrical activity that is going on in the brain. Scientists record brain waves using Electroencephalograph (EEG), which monitors electrical activity through electrodes placed on the scalp. There are four main types of brain waves: Alpha, Beta, Theta, and Delta. Sleep is just one of many types of consciousness we experience, and sleep itself comprises several states of consciousness. Even when we’re sleeping, our brains and bodies continue to work.
Alpha: very relaxed or meditating
Beta: awake and alert
Theta: lightly asleep
Delta: deeply asleep
Sleep is just one of many types of consciousness we experience, and sleep itself comprises several states of consciousness. Even when we’re sleeping, our brains and bodies continue to work.
Biological Rhythms
Sleep is affected by biological rhythms or periodic physiological changes. Biological rhythms are regular, periodic changes in a body’s functioning. There are three types of biological rhythms:
Circadian rhythms: biological cycles that occur about every 24 hours. Sleep follows a circadian rhythm. Hormone secretion, blood pressure, body temperature, and urine production also have circadian rhythms.
Infradian rhythms: Biological cycles that take longer than 24 hours. For example, women’s menstrual cycles occur about every 28 days.
Ultradian rhythms: biological cycles that occur more than once a day. Sleep follows an ultradian rhythm of about 90 minutes as well as a circadian rhythm. Alertness and hormone levels also follow ultradian rhythms.
Biological rhythms usually synchronize with environmental events such as changes in daylight. However, experiments have shown that many biological rhythms continue to have the same cycle even without cues from the environment. Such biological rhythms are endogenous, which means that they originate from inside the body rather than depend on outside cues.
Biological Clocks
Endogenous rhythms exist because the body has biological clocks that keep time. Biological clocks can be adjusted by environmental cues, such as changes in temperature.
In humans, the suprachiasmatic nucleus (SCN) is the main biological clock that regulates circadian rhythms of sleep. The SCN lies in the brain’s hypothalamus. When light stimulates receptors in the retina of the eye, the receptors send signals to the SCN. The SCN then sends signals to the nearby pineal gland, which secretes melatonin, a hormone that regulates the sleep cycle.
Disruptions of the Circadian Rhythm
The sleep/wake cycle is an important example of circadian rhythm. This cycle is regulated by the body’s internal clock, which responds to environmental cues such as light and temperature. When this rhythm is disrupted, it can cause a range of problems including fatigue and disorientation. Two common disruptions of the circadian rhythm are jet lag and shift work.
Jet lag is the fatigue and disorientation air travelers feel after a long flight. Although traveling itself drains energy, the time change also contributes to fatigue. People experience jet lag when the events in their environment are out of sync with their biological clocks.
Example: A traveler leaves New York City at 8:00 a.m. and arrives in London about seven hours later. For her, it’s 3:00 p.m., but because of the time change, in London it’s 8:00 p.m. Her body, thinking it’s mid-afternoon, will be confused by the lack of sunlight, and she’ll experience jet lag.
Shift work is another major disruption of the circadian rhythm. People who work night shifts or rotating shifts often have difficulty aligning their biological clocks with their work schedules. As a result, they may struggle to stay awake during work hours and experience difficulty sleeping during the day. Over time, shift work can lead to chronic fatigue, reduced alertness, and other health issues due to the constant misalignment of the circadian rhythm.
The Function of Sleep
Although everyone sleeps, no one really knows why people sleep. Researchers have proposed several theories to explain how sleep evolved to be a necessary behavior:
- People conserve energy by sleeping periodically.
- Sleep has a protective function, as it keeps people tucked away at night, safe from predators.
- Sleep restores body tissues that are depleted during daily activities.
Sleep Research
Sleep research has provided a lot of information about what happens to the brain and body during sleep. Researchers study sleep by monitoring subjects who spend the night in labs, and they use various instruments for different purposes:
Electroencephalographs (EEGs): record brain waves
Electromyographs (EMGs): record muscle activity
Electrooculograms (EOGs): record eye movements
Electrocardiographs (EKGs): record the activity of the heart
Other instruments monitor breathing, body temperature, and pulse.
Sleep Stages
During one night’s sleep, people pass through several cycles of sleep, each lasting about 90-100 minutes. There are four distinct stages of sleep in each cycle: 1, 2, 3 (formerly stages 3 and 4), and REM. The two major types of sleep are NREM sleep (non-rapid eye movement sleep) and REM sleep (rapid eye movement sleep).
Stages 1–3 (NREM Sleep)
When people are relaxed and ready to fall asleep, their EEG will show mostly alpha waves. When people fall asleep, they enter into stage 1 sleep which lasts just a few minutes. In stage 1, the EEG shows mostly theta waves. Heart rate, breathing rate, and body temperature drop, and muscles relax. People may experience hypnagogic sensations, such as fleeting images or the feeling of falling as they enter this initial stage. Stage 1 is very light sleep and is the first step into deeper stages of non-rapid eye movement (NREM) sleep.
After a few minutes of stage 1 sleep, people move into stage 2 sleep. Stage 2 lasts about 20 minutes and is characterized by short bursts of brain waves called sleep spindles and slower, larger waveforms called K-complexes. Stage 2 is deeper than stage 1, but people can still be easily awakened.
People then pass into slow-wave sleep, which occurs during stage 3. In this stage, which lasts 20-40 minutes or less as the night progresses, the EEG displays mostly delta waves. People in stage 3 sleep show slow pulse rates and slow breathing, have limp muscles, and are difficult to rouse.
REM Sleep
At the end of stage 3, people cycle back through stage 2 before entering REM sleep. In REM sleep, brain wave activity resembles that of an alert person, which is why REM sleep is also called paradoxical sleep.
During REM sleep, pulse rate and breathing become irregular, eyes move rapidly under closed lids, and muscles remain very relaxed. Genital arousal also happens during REM. In women, the clitoris becomes swollen with blood, and vaginal lubrication increases. In men, the penis becomes erect. EEGs show mostly beta waves during REM sleep. Although dreaming happens in other sleep stages as well, dreams are most vivid and frequent during REM sleep.
People typically go through about four to six sleep cycles during one night of sleep. The REM stage of sleep gets longer and longer as the night passes, while deep sleep (stage 3) gets shorter and shorter. During the night’s first sleep cycle, the REM stage lasts about 10 minutes. During the night’s last sleep cycle, people may spend about 40-60 minutes in REM sleep. Non-REM sleep becomes more shallow as the night goes on, and eventually the sleeper awakens.
Theories of Sleep
During sleep, the body undergoes essential restoration of resources needed for physical and mental functioning. NREM sleep plays a key role in restoring physical energy, repairing tissues, and strengthening the immune system, while REM sleep is important for memory consolidation, cognition, and emotional regulation. According to the restoration theory, sleep is necessary for repairing the wear and tear on the body that occurs throughout the day, helping to replenish energy levels, balance hormones, and regulate metabolic processes. Additionally, the memory consolidation theory suggests that sleep, particularly REM sleep, helps organize and solidify memories formed during waking hours, enhancing learning and retention. Without adequate sleep, cognitive functions like memory, attention, and problem-solving can suffer, and the body’s ability to recover from illness or injury is impaired. Energy conservation theory of sleep comes from the evolutionary perspective and states that sleep protects us and helps us adapt to our environments to aid survival.
Sleep Deprivation
Different people need different amounts of sleep. Some people can function with fewer than six hours of sleep a night, while others can’t manage without at least nine hours. Research shows that getting insufficient sleep can have negative effects on health, productivity, and performance.
Researchers have also studied the effects of insufficient REM sleep. Experiment subjects who are intentionally deprived of REM sleep tend to enter the REM stage of sleep more and more frequently during the night. After a REM-deprivation experiment has ended, subjects usually experience a REM rebound effect, spending more time in the REM stage on subsequent nights to make up for lost REM time.
Aging and Sleep
Sleep patterns change as people get older. Newborn babies spend about two-thirds of their time in sleep. As people age, they tend to sleep less. The amount of time spent in REM sleep also changes over time. In very young babies, about half of all sleep is REM sleep. As babies get older, the proportion of REM sleep decreases.
Sleep Disorders
Everyone has occasional difficulty sleeping, but some people have insomnia, a chronic problem with falling or staying asleep, which can lead to daytime fatigue and irritability. Another kind of sleep disorder is narcolepsy, which is a tendency to fall asleep periodically during the day. Narcolepsy can be dangerous, as people who experience it may fall asleep while driving or operating machinery.
Sleep apnea is another condition that can have negative effects on health and safety. People who have sleep apnea stop breathing many times during a night’s sleep, and each time they stop breathing, they wake up briefly and gasp for air. This prevents them from getting enough deep sleep, which leads to irritability and sleepiness during the day. Chronic sleep apnea can also result in high blood pressure.
REM sleep behavior disorder (RBD) is a condition where individuals physically act out their dreams during REM sleep due to a lack of the normal muscle paralysis that typically occurs in this stage. This can be dangerous, individuals with RBD may unintentionally hurt themselves or others during these episodes.
Somnambulism, or sleepwalking, occurs during deep sleep (slow-wave sleep) and involves walking or engaging in other activities while asleep. Most people in stage 3 sleep are still, quiet, and difficult to rouse. Sleepwalkers, however, sometimes become physically active during this stage of deep sleep. They may get up and walk around their room or even carry on a conversation, take a bath, cook, or go outside and get in their car. Because they are in a deep sleep, most sleepwalkers remember nothing of their actions when they wake up.
Dreams
The function of dreams is as much a mystery as the function of sleep. There are several theories as to why dreams occur, two of which are the activation-synthesis theory and the consolidation theory. The activation-synthesis theory proposes that neurons in the brain randomly activate during REM sleep. Dreams arise when the cortex of the brain tries to make meaning out of these random neural impulses. According to activation-synthesis theory, dreams are basically brain sparks.
The memory consolidation theory of dreams suggests that during sleep, especially REM sleep, the brain processes and organizes information learned throughout the day, thus helping to strengthen one’s memories. REM sleep, where most dreaming occurs, plays a critical role in consolidating new experiences and integrating them with existing knowledge. Research supporting this theory shows that sleep deprivation impairs memory and learning.
