The Fascinating Science Behind Sleep: Understanding the Different Stages and Cycles
Most people experience sleep as a gradual descent into unconsciousness that lasts through the night, but is that all sleep is? Does it hold any importance beyond simply filling a third of our lives? Why do we dream? Contrary to popular belief, sleep is not a uniform state. It comprises a combination of different states, including some that involve conscious awareness. Therefore, even when we sleep, we don't entirely lose consciousness.
The Different Stages of Sleep As we fall asleep, we first transition into a semi-wakeful state before gradually moving into four progressively deeper stages of sleep. During the night, we intermittently shift between these stages and a state where dreaming is common. This particular stage of sleep has a kind of conscious awareness unique to itself. Let's examine the different parts of the sleep cycle.
The Hypnagogic State Before slipping into a deep sleep, we often enter a hypnagogic state. It is a "twilight" state that's not quite daydreaming, but not yet dreaming. In this state, we begin to lose control over our body movements, our sensitivity to outside stimuli diminishes, and our thoughts become more fanciful and less bound by reality. Although this is a relaxing and enjoyable state for most, occasionally we're rudely awoken by a sudden jerk called a myoclonia.
Stages of Light and Deep Sleep Once we leave the hypnagogic state, we move through four progressively deeper stages of sleep. These stages are defined by electroencephalogram (EEG) measures of electrical brain activity. The depth of sleep constantly changes many times during the night. In fact, young adults shift in depth an average of 34 times in the first six hours of sleep. Therefore, sleep is not a static, uninterrupted state, but rather, it's a constantly changing one. The later stages of sleep involve deeper and slower frequencies of wavelength.
REM Sleep and Dreams In 1952, University of Chicago graduate student Eugene Aserinsky discovered rapid eye movements in a child he was monitoring during a sleepless night in Dr. Nathaniel Kleitman's laboratory. This discovery led to the hypothesis that the child's EEG resembled that of wakefulness during these periods of rapid eye movement because he was dreaming. Aserinsky and Kleitman then awakened many adult and child participants during this stage of sleep and found that over 80% of them reported dreaming. Rapid eye movement sleep, therefore, is a stage characterized by vivid dreams and unique conscious awareness. This provided a convenient way for scientists to know when dreams were occurring so that they could study them. Because of the characteristic eye
movements, this phase of sleep is referred to as rapid-eye-movement sleep or REM
sleep.
Autonomic storms occur during REM sleep, causing various changes in the body, including increased blood flow to the brain, irregular heartbeat, twitching muscles, and irregular breathing. The autonomic nervous system and other parts of the peripheral nervous system are also active during dreams. While voluntary control of large body muscles is lost during REM sleep, females experience vaginal lubrication and clitoral erection, and males experience penis erection. This fact is useful in diagnosing erectile dysfunction by determining whether the patient has erections during REM sleep. Laboratory studies have confirmed that REM sleep and autonomic storms are not limited to humans.
Dream frequency is often underestimated, with many people spending much more time in the world of dream consciousness than they realize. On average, college students spend about two hours a night in REM sleep, divided into four to six separate episodes. Based on reports of awakened sleepers, we know that we dream during at least 80% of these episodes of REM sleep. Young adults have around 30 to 40 REM dreams per week, although we do not remember dreaming this often when awake. Dreams during non-REM sleep are also common, with reports showing that people dream about half of the time when awakened during non-REM phases of sleep. While non-REM dreams are less bizarre and less negative, recent research suggests that dreams that occur during both REM and non-REM stages of sleep are more similar than different.
Non-REM sleep also contains dreams, contrary to initial beliefs by sleep researchers. The number of dreams during non-REM deep-wave sleep is much higher than previously suspected. Non-REM dreams are less bizarre and filled with less negative emotion than REM dreams, but the two types of dreams share many similarities. Beagles, like humans, experience autonomic storms during REM sleep and can be seen twitching and even making muffled howls.
Circadian Rhythms
The body operates on a circadian rhythm, a biological cycle of approximately 24 hours that regulates our pattern of wakefulness and sleep. This rhythm is controlled by parts of the brain stem and hypothalamus. The hormone melatonin is one key factor in regulating sleepiness, and its fluctuations follow a 24-hour pattern. The body also has many other circadian rhythms, most of which follow the pattern of the sleep-wake cycle.
The circadian sleep-wake cycle is influenced by the level of light, but surprisingly, it continues even when individuals are isolated in chambers that are always kept lighted. Disrupting circadian rhythms, such as during trips, can lead to a phenomenon known as "jet lag." People differ in how much they are affected by jet lag, but the time required to readjust to local time is generally longer when traveling from west to east. Data from 10 years of major league baseball showed that teams with the circadian advantage won more games than predicted by chance, due to the other team’s jet lag, gaining almost as much as the home team advantage.
People who rotate their work shifts also find it hard to ignore circadian rhythms. It is less disruptive to rotate from the night shift to the day shift or from the day shift to the swing shift than to rotate in the opposite direction. Recent studies point to the dangerous effects of disrupting our circadian rhythms, with negative consequences for physical well-being, including increased risk of accidents, cancers, and heart disease.
Sleep Disorders
Sleep is a natural process for everyone, but some individuals face significant difficulties with it. Sleep disorders are the common term used to describe these troublesome but highly treatable conditions. Insomnia is a type of sleep disorder where people report sleeping less than their desired amount. There are two primary types of insomnia, including sleep onset insomnia and early-awakening insomnia. In sleep onset insomnia, people find it challenging to fall asleep at their desired time, but they sleep normally once it begins. On the other hand, early-awakening insomnia is characterized by waking up earlier than desired, either several times in the middle of the night or early in the morning. These types of insomnia are common in individuals facing stress, anxiety, or depression, despite having no other psychological issues.
Narcolepsy is a rare sleep disorder, which affects less than one-half of 1% of the general population, but its impact can be severe. People with narcolepsy often unexpectedly fall into a deep slumber in the middle of work or even during conversations with others, especially when upset or stressed. They may also experience loss of muscle tone and show a lack of body movement as if suddenly falling into a dream sleep.
Sleep apnea is a sleep disorder characterized by the sudden, temporary interruption of breathing during sleep. Brief interruptions are normal, but people with sleep apnea experience more of these and of longer duration, sometimes occurring for 10 to 20 seconds at a time, hundreds of times per night. Sleep apnea is common in older or heavier adults who snore and can be caused either by too much relaxation of the muscles of the throat or by a temporary cessation of brain signals for breathing. Sleep apnea can lead to serious medical problems, but even moderate sleep apnea can have an impact on waking life, including tiredness, irritability, and impaired cognitive functioning.
In conclusion, sleep is much more complex than simply a period of unconsciousness that fills one-third of our lives. It consists of different stages, including some with conscious awareness and vivid dreams, and is regulated by our circadian rhythms. Dream frequency is often underestimated, with many people spending much more time in the world of dream consciousness than they realize. Disrupting circadian rhythms can lead to jet lag, which can have an impact on our physical and mental health. It is important to understand the importance of sleep and to prioritize getting enough rest to maintain our well-being.