Sleep: Understanding the Basics

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Facts on Sleep

  • Sleep is defined as a state of unconsciousness from which a person can be aroused, therefore, external stimuli have no effect. In this state, the brain is relatively more responsive to internal stimuli than external stimuli.
  • Sleep should be distinguished from coma. Coma is an unconscious state from which a person cannot be aroused.
  • Sleep is essential for the normal, healthy functioning of the human body. It is a complicated physiological phenomenon that scientists do not fully understand.
  • Historically, sleep was thought to be a passive state. However, sleep is now known to be a dynamic process and our brains are active during sleep.
  • Sleep affects our physical and mental health and is essential for the normal functioning of all the systems of our body, including the immune system. The effect of sleep on the immune system affects one's ability to fight disease and endure sickness.
  • Brain activity during sleep and wakefulness is a result of different activating and inhibiting forces that are generated within the brain. Neurotransmitters (chemicals involved in nerve signaling) control whether one is asleep or awake by acting on nerve cells (neurons) in different parts of the brain.
  • Neurons located in the brainstem cause sleep by inhibiting other parts of the brain that keep a person awake.

Importance of Sleep

Animal studies have shown that sleep is necessary for survival. The normal life span of rats is 2 to 3 years. However, rats deprived of sleep live for only about 3 weeks. They also develop abnormally low body temperatures and sores on their tails and paws. The sores probably develop because of impairment of the rats' immune systems.

In humans, it has been demonstrated that the metabolic activity of the brain decreases significantly after 24 hours of sustained wakefulness. Sleep deprivation results in a decrease in body temperature, a decrease in immune system function as measured by white blood cell count (the soldiers of the body), and a decrease in the release of growth hormone. Sleep deprivation can also cause increased heart rate variability.

For our nervous systems to work properly, sleep is needed. Sleep deprivation makes a person drowsy and unable to concentrate the next day. It also leads to impairment of memory and physical performance and reduced ability to carry out mathematical calculations. If sleep deprivation continues, hallucinations and mood swings may develop.

Release of growth hormone in children and young adults takes place during deep sleep. Most cells of the body show increased production and reduced breakdown of proteins during deep sleep. Sleep helps humans maintain optimal emotional and social functioning while we are awake by giving rest during sleep to the parts of the brain that control emotions and social interactions.

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Stages of Sleep

As mentioned earlier, sleep is a dynamic process. There are two distinct states that alternate in cycles and reflect differing levels of neuronal activity. Each state is characterized by a different type of brain wave (electrical activity that is recorded with the help of electrodes placed on the skull) activity. Sleep consists of nonrapid eye movement (NREM) and R sleep (rapid eye movement)

  • Stage I (light sleep)
  • Stage II
  • Stage III (deep sleep)

The stages of NREM sleep and R or REM sleep cycle over and over again during a night's sleep. Stages I, II, III, and IV are followed by REM sleep. A complete sleep cycle, from the beginning of stage I to the end of REM sleep, usually takes about one and a half hours.

For the purpose of analysis, a night's sleep is divided into three equal time periods: Sleep in the first third of the night, which comprises the highest percentage of NREM; sleep in the middle third of the night; and sleep in the last third of the night, the majority of which is REM. Awakening after a full night's sleep is usually from REM sleep.

NREM Sleep

Stage I is a stage of light sleep and is considered a transition between wakefulness and sleep. During this stage, the muscles begin to relax. It occurs upon falling asleep and during brief arousal periods within sleep, and usually accounts for 5% to 10% of total sleep time. An individual can be easily awakened during this stage.

Stage II occurs throughout the sleep period and represents 40% to 50% of the total sleep time. During stage II, brain waves slow down with occasional bursts of rapid waves. Eye movement stops during this stage.

In stage III, extremely slow brain waves called delta waves begin to appear. They are interspersed with smaller, faster waves. This stage represents about 20% of total sleep time. Stages III is sometimes called deep sleep, during which all eye and muscle movement ceases. It is difficult to wake up someone during these two stages. If someone is awakened during deep sleep, he does not adjust immediately and often feels groggy and disoriented for several minutes after waking up. Some children experience bedwetting, night terrors, or sleepwalking during deep sleep.

REM Sleep or R Sleep

REM sleep represents 20% to 25% of the total sleep time. REM sleep follows NREM sleep and occurs four to five times during a normal 8- to 9-hour sleep period. The first REM period of the night may be less than 10 minutes in duration, while the last may exceed 60 minutes. In a normal night’s sleep, bouts of REM occur every 90 minutes.

When the person is extremely sleepy, the duration of each bout of REM sleep is very short or it may even be absent. REM sleep is usually associated with dreaming. During REM sleep, the eyeballs move rapidly, the heart rate and breathing become rapid and irregular, and the blood pressure rises. The muscles of the body are virtually paralyzed. The brain is highly active during REM sleep, and the overall brain metabolism may be increased by as much as 20%. The electrical activity recorded in the brain during REM sleep is similar to that which is recorded during wakefulness.

Sleep Apnea

Sleep apnea means that your breathing often is blocked or partly blocked during sleep. The problem can be mild to severe, based on how often your lungs don't get enough air. This may happen from 5 to more than 50 times an hour.

This topic focuses on obstructive sleep apnea, which is the most common type.

A less common type of apnea, called central sleep apnea, can occur in people who have had a stroke, have heart failure, or have a brain tumor or infection. Even though this topic isn't about central sleep apnea, some of the treatments discussed here may also help treat it. Talk with your doctor to find out more about central sleep apnea.

SOURCE: Healthwise

Sleep at Different Stages of Life

Infancy

Infants have an overall greater total sleep time than any other age group. Their sleep time can be divided into multiple periods. In newborns, the total sleep duration in a day can be 14 to 16 hours. Over the first several months of life, sleep time decreases; by age 5 to 6 months, sleep consolidates into an overnight period with at least one nap during the day.

REM sleep in infants represents a larger percentage of the total sleep at the expense of stage III. Until age 3 to 4 months, newborns transition from wakefulness into REM sleep. Thereafter, wakefulness begins to transition directly into NREM sleep.

Adulthood

In adults, sleep of 8 to 8.4 hours is considered fully restorative. In some cultures, total sleep is often divided into an overnight sleep period of 6 to 7 hours and a nap of 1 to 2 hours.

Some people may need as little as 5 hours or as much as 10 hours of sleep every day. The period of time a person sleeps depends also on the fact whether he or she has been deprived of sleep in previous days. Sleeping too little creates a "sleep debt." This debt needs to be adjusted by sleeping for longer periods over the next few days. People who sleep less have an impairment of judgment and reaction time.

Old age

People tend to sleep more lightly and for shorter periods as they get older. In elderly persons, the time spent in stage III decreases by 10% to 15%, and the time in stage II increases by 5% compared to young adults, representing an overall decrease in total sleep duration.

Time taken to fall asleep and the number and duration of overnight arousal periods increase. Thus, to have a fully restorative sleep, the total time in bed must increase. If the elderly person does not increase the total time in bed, complaints of insomnia and chronic sleepiness may occur.

Sleep fragmentation results from the increase in overnight arousals and may be exacerbated by the increasing number of medical conditions related to old age, including sleep apnea (interrupted breathing during sleep), musculoskeletal disorders, and cardiopulmonary disease.

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Circadian Rhythms That Influence Sleep

Biological variations that occur in the course of 24 hours are called circadian rhythms. Circadian rhythms are controlled by the body's biological clock. Many bodily functions follow the biologic clock, but sleep and wakefulness comprise the most important circadian rhythm. Circadian sleep rhythm is one of the several body rhythms modulated by the hypothalamus (a part of the brain).

Light directly affects the circadian sleep rhythm. Light is called a "zeitgeber," a German word meaning time-giver, because it sets the biological clock. A practical purpose has been proposed for the circadian rhythm, using the analogy of the brain being somewhat like a battery charging during sleep and discharging during wakefulness.

Body temperature cycles are also under control of the hypothalamus. An increase in body temperature is seen during the course of the day and a decrease is observed during the night. The temperature peaks and troughs are thought to mirror the sleep rhythm. People who are alert late in the evening (evening types) have body temperature peaks late in the evening, while those who find themselves most alert early in the morning (morning types) have body temperature peaks early in the evening.

Melatonin (a chemical produced by the pineal gland in the brain) has been implicated as a modulator of light entrainment. It is secreted maximally during the night. Prolactin, testosterone, and growth hormone also demonstrate circadian rhythms, with maximal secretion during the night.

Circadian rhythms can be affected to a certain degree by almost any kind of external stimulus, for example, the beeping of the alarm clock or the timing of meals. When we cross time zones, our circadian rhythms get disrupted leading to jet lag. It usually takes several days for our body rhythms to adjust to the new time.

Symptoms similar to those seen in people with jet lag are common in people who work during nights or work in shifts. Because these people's wake time conflicts with powerful sleep-regulating cues like sunlight, they often become uncontrollably drowsy during work or may have difficulty falling asleep during their off time. Their biological clock wants to do one thing while they are doing something entirely different. People working in shifts have an increased risk of heart, gastrointestinal, emotional, and mental problems. All these problems may be related to the disruption of the circadian sleep rhythm.

Substances That Alter Sleep

Sleep and wakefulness are influenced by different neurotransmitters in the brain. Some substances can change the balance of these neurotransmitters and affect our sleep and wakefulness. Caffeinated drinks (for example, coffee) and medicines (for example, diet pills) stimulate some parts of the brain and can cause difficulty in falling asleep. Many drugs prescribed for the treatment of depression suppress REM sleep.

People who smoke heavily often sleep very lightly and have reduced duration of REM sleep. Heavy smokers tend to wake up after 3 or 4 hours of sleep due to nicotine withdrawal. Some people who have insomnia may use alcohol. Even though alcohol may help people to fall into light sleep, it deprives them of REM sleep and the deeper and more restorative stages of sleep. Instead, it keeps them in the lighter stages of sleep from which they can be awakened easily.

During REM sleep, we lose some of our ability to regulate our body temperature. Therefore, abnormally hot or cold temperatures can disrupt our REM sleep. If our REM sleep is disturbed, the normal sleep cycle progression is affected during the next sleeping time and there is a possibility of slipping directly into REM sleep and going through long periods of REM sleep until the duration of REM sleep that is lost is caught up.

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Sleep Deprivation

Because the function of sleep has not been fully determined, the exact number of hours that a person should sleep is unknown. Some persons claim to work optimally with only 3 to 5 hours of sleep per night, while some admit needing at least 8 hours of sleep per night (or more) to perform effectively. Therefore, sleep deprivation is best defined by group means and in terms of the tasks impaired.

In tasks requiring judgment, increasingly risky behaviors emerge as the total sleep duration is limited to 5 hours per night. The high cost of an action is seemingly ignored as the sleep-deprived person focuses on limited benefits. These findings can be explained by the fact that metabolism in the prefrontal and parietal associational areas of the brain decrease in individuals deprived of sleep for 24 hours. These areas of the brain are important for judgment, impulse control, attention, and visual association.

Sleep deprivation is a relative concept. Small amounts of sleep loss (for example, 1 hour per night over many nights) produce subtle cognitive impairment, which may go unrecognized. More severe restriction of sleep for a week leads to profound cognitive deficits, which may also go unrecognized by the individual. If you feel drowsy during the day, fall asleep for very short periods of time (5 minutes or so), or regularly fall asleep immediately after lying down, you are probably sleep-deprived.

Many studies have made it clear that sleep deprivation is dangerous. With decreased sleep, higher-order cognitive tasks are impaired early and disproportionately. On tasks used for testing coordination, sleep-deprived people perform as poorly as or worse than people who are intoxicated. Total sleep duration of 7 hours per night over 1 week has resulted in decreased speed in tasks of both simple reaction time and more demanding computer-generated mathematical problem solving. Total sleep duration of 5 hours per night over 1 week shows both a decrease in speed and the beginning of accuracy failure.

Total sleep duration of 7 hours per night over 1 week leads to impairment of cognitive work requiring simultaneous focus on several tasks. In driving simulations, for example, accidents increase progressively as total sleep duration is decreased to 7, 5, and 3 hours per night over 1 week. Driver fatigue is responsible for over 100,000 motor vehicle crashes and 1,500 deaths each year, according to the National Highway Traffic Safety Administration.

Since drowsiness occurs just before falling asleep, driving while drowsy often leads to disaster.

According to the National Sleep Foundation "If you have trouble keeping your eyes focused, if you can't stop yawning, or if you can't remember driving the last few miles, you are probably too drowsy to drive safely." It is important to know that caffeine and other stimulants cannot overcome the effects of severe sleep deprivation. Therefore, if you find yourself driving in a sleep-deprived state, it is imperative that you find a safe place to stop and catch up on your sleep before continuing safely on your way.

How to Fall Asleep Fast

Falling asleep is step one of getting a restful night so that the body can recover from daily activity. To fall asleep optimally requires that both emotional and physical states be ready to rest. This means minimizing stressful situations prior to sleep, being comfortable in the sleeping environment in regard to the senses (for example, noise, smells, temperature, and pressure from bedding), trying to go to sleep regularly at approximately the same time, and getting appropriate exercise activity during the course of the day. Also, for ideal total sleep, it is helpful to avoid lifestyle activities that aggravate proper sleep, such as smoking and alcohol.

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Reviewed on 11/20/2017

Medically reviewed by Peter O’Connor, MD; American Board of Otolaryngology with subspecialty in Sleep Medicine

REFERENCE:

"Classification of sleep disorders." UptoDate.

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