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Biology 103
2001 Second Web Report
On Serendip
 
 

Nod:  A Tourist's Handbook

Jessica Blucher


Introduction

WordNet (r) 1.6 (1) defines a sleep as follows:

sleep
n 1: a natural and periodic state of rest during which consciousness of the world is suspended; "he didn't get enough sleep last night"; "calm as a child in dreamless slumber" [syn: {slumber}]
3: a period of time spent sleeping; "he felt better after a little sleep"; "a brief nap" [syn: {nap}]
v 1: be asleep [syn: {kip}, {slumber}, {log Z's}, {catch some Z's}] [ant: {wake}]

 For college students, sleep is viewed either as a necessary evil that keeps them from their work or a goal that they can rarely attain.  On the rare occasions that I have been fortunate enough to experience a full night of sleep, I have awakened alert and well rested.

Who sleeps?

 All mammals and birds sleep, but scientists are unsure if reptiles, fish, insects, and other life forms sleep.  Total sleep amounts differ greatly across species.  In general, large mammals tend to sleep less than small mammals.  The giraffe and elephant, for instance, sleep only 2 to 4 hours a day, while bats, opossums, and armadillos sleep 18 hours a day or more.  (2)

How do they sleep?

 While sleeping, most animals close their eyes and adopt particular positions referred to as sleep postures.  Humans typically lie down to sleep, for example, while giraffes kneel and bend their long necks around to rest their heads in the crook of their hind knee.  Some animals, such as dolphins, can sleep while they are moving.  (2)

How much does the average human sleep?

 The average person spends about 8 hours a day, 56 hours a week, 224 hours a month and 2,688 hours a year sleeping.  (3)

What happens when we are asleep?

 It's true that when we're asleep it doesn't look like much is happening, and so that is what people believed for a long time.  If fact, until the 1950s, most people thought of sleep as a passive, dormant part of our daily lives.  (4)  Since then, research has been done on how the brain operates during sleep mode.

How do scientists do that?

 Scientists measure sleep by placing metal electrodes on the scalp to record the electrical activity of the brain.  This procedure, called electroencephalography (EEG), enables sleep researchers to evaluate levels of brain activity at different times during sleep.  Researchers use similar electrodes to record a sleeping person's body muscle activity and rate of eye movement.  (2)

What did they find out?

 In reality, there is no one ‘sleep mode'.  There are two basic forms of sleep: slow wave sleep (SWS) [also known as NREM] and rapid eye movement (REM) sleep.  Slow wave  sleep is actually 4 different stages of sleep (Stage 1, Stage 2, Stage 3 and Stage 4) with different EEG patterns.  (3)  As we sleep, our brains cycles through these stages about 4-5 times.  (3)

What is REM sleep?

 REM sleep is the fifth part of the sleep cycle.  This is where dreaming occurs.

 REM sleep is generated by a region in the brainstem, called the pons, and adjacent portions of the midbrain.  Researchers have found that chemical stimulation of the pons will induce very long periods of REM sleep, while damage or injury to this brain region can greatly reduce or even prevent REM sleep.  Animal studies have found that some neurons within the pons and midbrain are active only in REM sleep while other neurons in this region are entirely inactive only during REM sleep.  (2)

 The first REM sleep period usually occurs about 70 to 90 minutes after we fall asleep.  A complete sleep cycle takes 90 to 110 minutes on average.  The first sleep cycles each night contain relatively short REM periods and long periods of deep sleep.  As the night progresses, REM sleep periods increase in length while deep sleep decreases.  By morning, people spend nearly all their sleep time in stages 1, 2, and REM.  (4)

 Together, these neurons control muscle tone and other aspects of REM sleep.  In REM sleep, most muscles in the body are turned off.  This lack of muscle tone, called atonia, is particularly complete in the muscles of the back, neck, arms, and legs.  Less affected are the muscles that move the eyes and the muscles responsible for breathing.  (2)

 The combined effect of the sleep-active and sleep-inactive neurons explains why sleepers do not physically act out the vivid dreams they have during REM sleep and instead only twitch or make small movements.  Humans with malfunctioning REM sleep-active and REM sleep-inactive systems thrash around in their sleep, often punching their bedmates or hurting themselves as they act out their dreams.  (2)

How much sleep do we need?

 Amounts of sleep vary significantly with age and even between individuals.  The amount of sleep each person needs depends on many factors, including age.  Newborns sleep the most—a newborn baby sleeps between 17 and 18 hours a day, spending nearly half of that time in REM sleep.  Teenagers need about 9 hours on average.  For most adults, 7 to 8 hours a night appears to be the best amount of sleep, although some people may need as few as 5 hours or as many as 10 hours of sleep each day.  People tend to sleep more lightly and for shorter time spans as they get older, although they generally need about the same amount of sleep as they needed in early adulthood.  Therefore the elderly spend less time in deep NREM sleep, and their sleep is more easily interrupted.  Women in the first 3 months of pregnancy often need several more hours of sleep than usual.  The amount of sleep a person needs also increases if he or she has been deprived of sleep in previous days.  Getting too little sleep creates a "sleep debt," which is much like being overdrawn at a bank.  Both REM and NREM sleep decrease with age, and by age five, children sleep between 10 and 12 hours a day, spending about 20 percent of that time in REM sleep.  (2), (4)

 Eventually, your body will demand that the debt be repaid.  We don't seem to adapt to getting less sleep than we need; while we may get used to a sleep-depriving schedule, our judgment, reaction time, and other functions are still impaired.  (4)

 Experts say that if you feel drowsy during the day, even during boring activities, you haven't had enough sleep.  If you routinely fall asleep within 5 minutes of lying down, you probably have severe sleep deprivation, possibly even a sleep disorder.  Microsleeps, or very brief episodes of sleep in an otherwise awake person, are another mark of sleep deprivation.  In many cases, people are not aware that they are experiencing microsleeps.  The widespread practice of "burning the candle at both ends" in western industrialized societies has created so much sleep deprivation that what is really abnormal sleepiness is now almost the norm.  (4)

 Many studies make it clear that sleep deprivation is dangerous.  Sleep-deprived people who are tested by using a driving simulator or by performing a hand-eye coordination task perform as badly as or worse than those who are intoxicated.  Sleep deprivation also magnifies alcohol's effects on the body, so a fatigued person who drinks will become much more impaired than someone who is well-rested.  Driver fatigue is responsible for an estimated 100,000 motor vehicle accidents and 1500 deaths each year, according to the National Highway Traffic Safety Administration.  Since drowsiness is the brain's last step before falling asleep, driving while drowsy can ? and often does ? lead to disaster.  Caffeine and other stimulants cannot overcome the effects of severe sleep deprivation.  The National Sleep Foundation says that 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.  (4)

 REM sleep amounts also vary across animal species, depending on the size of the animal and its level of development at birth.  The size of an animal seems to affect the type of sleep it experiences—small animals tend to spend more time in REM sleep.  Animals that are born in relatively helpless states, such as opossums and humans, generally have more REM sleep as newborns than animals that can hunt, eat, keep warm, and defend themselves soon after birth, for instance guinea pigs or horses.  Even as animals age into adulthood, those born relatively immature continue to spend more time in REM sleep than animals that are mature at birth.  (2)

 One of the myths about sleep is that smarter animals spend longer periods in REM sleep.  REM sleep amounts in humans or primates—believed to be the most intelligent members of the animal kingdom—are not remarkably high or low.  Rather, they seem to fit the general rule of level of maturity at birth.  REM sleep amounts in whales and dolphins—animals also recognized for their high intelligence—are among the lowest seen in any mammal.  These animals, both born relatively mature, also seem to fit the general rule relating REM sleep amounts to level of maturity at birth.  (2)

So why do we sleep, anyway?

 Although no one knows for sure why we sleep, there are a number of theories.  Sleep may have evolved to protect animals from their predators by reducing their activity during the times when they are most vulnerable.  (2)

 Although scientists are still trying to learn exactly why people need sleep, animal studies show that sleep is necessary for survival.  For example, while rats normally live for two to three years, those deprived of REM sleep survive only about 5 weeks on average, and rats deprived of all sleep stages live only about 3 weeks.  Sleep-deprived rats also develop abnormally low body temperatures and sores on their tail and paws.  The sores may develop because the rats' immune systems become impaired.  Some studies suggest that sleep deprivation affects the immune system in detrimental ways.  (4)

 Sleep appears necessary for our nervous systems to work properly.  Too little sleep leaves us drowsy and unable to concentrate the next day.  It also leads to impaired memory and physical performance and reduced ability to carry out math calculations.  If sleep deprivation continues, hallucinations and mood swings may develop.  Some experts believe sleep gives neurons used while we are awake a chance to shut down and repair themselves.  Without sleep, neurons may become so depleted in energy or so polluted with byproducts of normal cellular activities that they begin to malfunction.  Sleep also may give the brain a chance to exercise important neuronal connections that might otherwise deteriorate from lack of activity.  (4)

 Deep sleep coincides with the release of growth hormone in children and young adults.  Many of the body's cells also show increased production and reduced breakdown of proteins during deep sleep.  Since proteins are the building blocks needed for cell growth and for repair of damage from factors like stress and ultraviolet rays, deep sleep may truly be "beauty sleep." Activity in parts of the brain that control emotions, decision-making processes, and social interactions is drastically reduced during deep sleep, suggesting that this type of sleep may help people maintain optimal emotional and social functioning while they are awake.  A study in rats also showed that certain nerve-signaling patterns which the rats generated during the day were repeated during deep sleep.  This pattern repetition may help encode memories and improve learning.  (4)

 Research has shown that REM and NREM sleep may serve specific biological functions.  Sleep deprivation studies reveal that humans and other animals respond to sleep loss in the same way.  When study subjects are deprived of REM sleep, they tend to spend longer periods in REM sleep during their next sleeping period to make up for the loss.  REM sleep after deprivation is more intense, with more eye movements per minute than in normal REM sleep.  Similarly, subjects deprived of NREM sleep usually spend more time in NREM sleep afterward.  EEGs measuring brain activity show that this rebound NREM sleep also differs from normal NREM sleep.  This research suggests that the body needs adequate levels of both REM and NREM sleep.  This conclusion has led many sleep researchers to believe that the two kinds of sleep serve different biological purposes, although the exact functions remain unclear.  (2)

 The relationship between maturity at birth and REM sleep suggests that REM sleep plays a role in the development of the brain.  REM sleep may have a related function later in life as well.  However, that function remains a mystery. (2)

WWW Sources

1)The DICT Development Group

2)Center for Sleep Research, Siegel Lab, on the UCLA website

3)Neuroscience for Kids - Sleep, on the University of Washington website

4)Understanding Sleep:  Brain Basics, on the National Institute of Neurological Disorders and Stroke website


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