Introduction

Throughout the centuries, sleep has been recognized as a universal, essential human need.  The early Greeks distinguished human beings from the deities on the basis of the need for sleep citing the eternal vigilance of the gods versus the faltering of human beings in sleep 1.  Even until the late 1800s, the topic of sleep remained largely in the realm of religion, art, poetry, and folklore 2.  The modern study of sleep actually began approximately 50 years ago with the publication of a seminar paper in Science describing sleep as a cyclic process characterized by episodic bursts of rapid eye movements 3.  Since that time, a wealth of information has been uncovered regarding this phenomenon and sleep medicine has become an empirically supported clinical specialty.

Normal Sleep

Sleep was originally thought to be a simple, passive, and uniform phenomenon.  Today, sleep is regarded as an active and complex state characterized by several rhythmic stages and cycles that form characteristic patterns in individuals and groups 4.   The structure and timing of the sleep stages and cycles, collectively known as "sleep architecture," can be studied objectively using a technique known as polysomnography (PSG). PSG recordings involve the simultaneous monitoring of electrical activity associated with brain waves, muscle contraction, eye movements, and the heart. Respiratory patterns, and blood oxygen levels are also frequently measured. 

PSG has revealed that there are two kinds of sleep: non-rapid-eye-movement (NREM) and rapid-eye-movement (REM) sleep.   NREM sleep includes four stages, the final two collectively referred to a slow-wave sleep (SWS).  Other measures made during a PSG include time taken to fall asleep; time taken to get to the various sleep stages; percent of time spent in each of the stages; total sleep time; time spent sleeping while in bed; and number of awakenings throughout the night 4,5. 

NREM stage 1 is a transitional phase between full wakefulness and sleep and is characterized by small-sweeping brain waves with mixed-frequency,  as well as a level of muscle tone somewhat lower than that of relaxed wakefulness (see Fig. 1).  Some individuals experience sleep jerks of the face, hands, or feet.  During this stage, reactivity to outside stimuli is diminished, mental processes change, thoughts begin to drift, thinking is less "reality-oriented", and short dreams often develop.  Many people feel that they are awake during this stage as they are easily aroused.  In normal sleepers, stage 1 lasts from about one to seven minutes.

Stage 2 is marked by distinct changes in brain wave activity, including the appearance of "sleep spindles” or brief bursts of electrical activity, increased numbers of certain types of wave forms (see Fig. 1).   Muscle tone is only slightly further reduced, eye movements are absent, and mental activity consists of short, mundane, and fragmented thoughts.  Stage 2—which lasts five to fifteen minutes—is classified by most sleep researchers as the first bona fide sleep stage 5.

Stages 3 and 4 together (SWS) are distinguished by large-sweeping, slow-frequency waves, (see Fig. 1).  Of the NREM stages, this is the deepest and random stimuli will usually not arouse the individual.  As in stage 2, eye movements are usually absent during stages and muscular activity remains at a decreased level compared to wakefulness.  During SWS, the cardiac and respiratory rates, metabolic rates, and blood pressure decrease to baseline levels.   Growth hormone is secreted by the pituitary gland, and levels of other constructive hormones, such as prolactin and testosterone, are high 6.  Thus, many believe that NREM sleep is a time of energy conservation, body renewal, and tissue building. 

 REM sleep resembles NREM stage 1 with large-sweeping, mixed-frequency brain waves (see Fig. 1).  However, there are bursts of jerky movements of both eyes and these give REM sleep its name.   The skeletal muscles are paralyzed during REM sleep and it is believed that this phenomenon is designed to prevent the sleeper from acting out dreams.   Blood pressure, heart rate, cardiac output, and respiratory rate become erratic.  Oxygen consumption and blood flow to the brain increase.   In about 80 percent of awakenings from REM sleep, people recall dreams, whereas only about 5 percent of NREM awakenings result in dream reports.  Thus, some theorists believe that this type of sleep is extremely important to psychological and emotional well-being 7.

Sleep usually progresses through repetitive cycles beginning with NREM stages 1 though 4, back again to stage 2, and subsequently into REM (see Fig. 2).  These cycles occur approximately every 70 to 120 minutes in the adult (50 minutes in the infant), with four to five cycles normally completed during a sleep period.  NREM sleep dominates the first third of the sleep period, whereas REM periods increase in length later in the sleep period.  This may explain why morning naps are frequently composed of REM sleep while those in the afternoon are more often NREM. The actual duration of each stage may vary under certain conditions such as increased age, prior sleep debt, medication usage, and circadian rhythm disturbances (such as jet lag).  After prolonged wakefulness, SWS increases during the first night with recovery of REM sleep postponed to the second or third night.                    

Sleep can also be studied through subjective assessments including time taken to fall asleep, number of awakenings, depth and length of sleep, refreshing quality of sleep, satisfaction with sleep and soundness of sleep.  This type of information can be collected through the use of sleep questionnaires, sleep diaries, visual analogue scales, and interviews 4,5.  Subjective assessments of sleep quality do not always correlate with the objective data obtained through PSG recordings.  However, personal satisfaction with sleep and feeling refreshed after a sleep period are variables that are extremely significant in the assessment of sleep patterns.

Observation is an important sleep measurement technique and is considered the “gold standard” for sleep monitoring in infants 8.  REM sleep was first discovered by observing the eye movements of infants that occurred at regular intervals during sleep. 1 Typically individuals who are sleepy or sleep deprived show characteristic waking behaviors including yawning, eye rubbing, head nodding, drooping of the eyelids, irritability, and slowed movement 9.   Other observable waking behaviors that can be noted in sleep alterations include automatic behavior (purposeful but inappropriate behavior during the day such as putting dishes in the washing machine), unintentional sleep episodes, sudden muscle weakness, and sleep drunkenness.   Although lying quietly in a horizontal position is typical during sleep, movements and position changes can occur and are considerable a normal part of sleep behavior.  Abnormal sleep-related behaviors include bizarre postures, restless sleep, jumping and jerking of the extremities, seizure activity, and dream enactment.  Video-recordings of these behaviors during PSG are often made to assist in the assessment and diagnosis of the sleep problem 5.

The Function of Sleep

Major controversy still exists over the exact function of sleep.  Many sleep theorists believe that sleep is crucial for bodily and mental restoration 10,11,12 and for energy conservation 13.  Other theorists propose that substances from bodily fluids accumulate in the blood during waking, inducing tiredness and are then removed during sleep 14.  Horne 15,16 suggests that sleep is a state of decreased activity and protein conservation caused by the lack of food intake during the night.  Others have proposed integrated theories with many of these components 17.  Some have wondered if sleep is really necessary at all 18.  

Sleep deprivation studies have been used extensively to examine the relationship of sleep to health and well-being.  Early studies demonstrated that keeping healthy young subjects awake for an entire night makes them very sleepy but has almost no effect on their next-day performance 19,20. Even after 10 days of total sleep deprivation, two or three nights of extended sleep usually return a normal volunteer to baseline.  However, other studies have demonstrated that SWS deprivation is associated with fatigue, anxiety, increased illness, increased sensitivity to pain, and a decreased immune response. Symptoms of REM sleep deprivation included restlessness, disorientation, combativeness, delusion, and hallucinations 21,22.  Animal studies have demonstrated that total sleep deprivation for periods of 2 to 3 weeks ultimately leads to death 23,24.  Due to conflicting reports, it is still widely believed that the overwhelming tiredness which occurs in humans in response to sleep deprivation is more striking than the biochemical, emotional, or behavioral changes that take place.                

According to McGinty and Szqmusiak 25, studies with birds and mammals have led to the identification of specialized areas in the brain that network and integrate temperature and sleep controls.  As brain temperature increases, SWS is facilitated and in turn facilitates heat loss through expansion of blood vessels and reduction of the metabolic rate.   In addition, many immune factors have been shown to promote SWS.  These substances are also fever-producing.  Thus, this theory provides a physiologic explanation of why people become sleepy when having fevers and infections.  In summary, the theory proposes that SWS, temperature regulation, and immune function have important linkages.

Although recent research seems promising in uncovering some of the mysteries of sleep, much work remains to be done.  Until such time that more is known, it seems reasonable to assume that, at the very least, sleep promotes health and healing though improving morale and well-being.  However, it may also play a crucial role in tissue building, temperature regulation, immune function, and other processes vital to maintaining stability of normal body states.



References


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