Sleep is a mysterious and bizarre
concept that has puzzled many people for centuries. Why do our bodies spend so
much time inefficiently when we sleep? Yes, we need sleep to function, but what
are our bodies really doing? Does our body just replenish energy during the
precious time we spend asleep?
During one night, our bodies
transition through four stages and two types of sleep. The four stages occur in
cycles and the two types of sleep are NREM (non-rapid eye movement) sleep and
REM (rapid eye movement) sleep. As we fall into unconsciousness, our bodies
begin the sleep cycle with Stage 1. Stage 1 normally lasts five to ten minutes
and is the transition between consciousness and sleep. It is also the stage where
one may experience hallucinations, the feeling of falling, or floating
weightlessly. The brain will begin to produce sleep spindles during Stage 2,
which are rapid, rhythmic brain waves.1 Stage 2 lasts for about 20
minutes, during which body temperature decreases and heart rate begins to slow.
Following Stage 2 is delta sleep, or Stage 3. Here the brain starts to produce
deep, slow brain waves called delta waves – hence the name of Stage 3. As Stage
1 was the transitional period between consciousness and sleep, Stage 3 is the
transitional period between light sleep and deep sleep. Up until the end of
Stage 3, our bodies have been in NREM sleep. As Stage 3 concludes, the body
begins to enter REM sleep.2 REM sleep characteristics can include
the fluttering of eyes under the eyelids, irregular and shallow breathing, and
loss of muscle control. REM sleep occurs about an hour after one first falls
asleep and is also called Stage 4. The body is still and relaxed, save for a
few occasional twitches, and blood pressure rises. This is also the stage in
which dreaming will occur.3
The structure of sleep follows a
pattern alternating between NREM sleep and REM sleep. The body usually goes
through Stage 1 once throughout the night, passing through Stages 2 and 3 and
finally entering REM sleep. After about 10 minutes of REM sleep, the body will
return to Stage 2 and repeat this cycle throughout the night. The first time
your body enters REM sleep, the stage will only last about ten minutes but by
the end of the night, the last cycle of REM sleep can be up to 60 minutes long.
For every cycle of sleep the body experiences, the length of REM sleep will
increase while the length of delta sleep will decrease. By the end of the
night, there is almost no delta sleep left in the cycle. Each cycle takes
approximately 90 minutes and the body will go through the cycle of stages four
to five times a night. When you awaken naturally, you will have just finished a
period of REM sleep.1
Sleep is vital to human health. When
our bodies fall into unconsciousness, they do a lot more than what it looks
like they’re doing. Although it doesn’t seem like it, the amount of energy
consumed by the brain does not decrease when the body enters sleep, primarily
because of several systems in the brain active during sleep.
The first is a system that flushes
waste from the brain. During consciousness, by-products of neural activity
build up and every night sleep clears this build up. Since the brain is
enclosed by a set of molecular gateways – the blood-brain barrier – the system
that clears waste in the body does not extend to the brain. During sleep, cerebral
spinal fluid (CSF) is pumped through the brain’s tissue and the waste is then
flushed back into the circulatory system where it eventually works its way to
the liver. As this system cleans the brain, brain cells shrink to allow CSF to
flow more smoothly through tissue. This system is called the glymphatic system
and is ten times more active during sleep versus consciousness.4
Another active region of the brain
during sleep is waves produced during different stages of sleep. These
different waves are characterized by frequencies corresponding to the nature of
the stage it is released in. These waves show the amount of activity in the
brain and our level of consciousness.
In Stage 1 and REM sleep, the brain
produces theta waves which are usually measured at 4 to 7.5 cycles per second,
or 4 to 7.5 hertz. These waves are also experienced during deep meditation.
Theta state heightens receptivity and can be produced fleeting as the body
wakes or falls asleep. In addition, during REM sleep, the brainstem blocks
information from leaving the brain’s motor cortex so your muscles are relaxed
and unmoving.5 Throughout Stage 2, sleep spindles will occur
periodically as they are rhythmic waves and unvarying in form. The sleep
spindles are measured between 10 and 14 hertz.6 In Stage 3, delta
waves are produced. Their frequency ranges from 0 hertz to 4 hertz and they are
the lowest set of frequencies a human brain will experience. Certain
frequencies of delta waves trigger the release of growth hormone and are essential
to the restorative process of sleep.5 These active portions of the
brain during sleep contribute to the remedial nature of sleep. This means that
our time sleeping is absolutely critical to our performance; the human body is
actively working and cleansing itself when we are not aware.
Throughout
a night of sleep, dreams are perhaps the most least understood stage of
unconsciousness. What are their purposes and what do they mean? Although there
are no solid facts on the purposes of dreams, there are many theories as to why
dreaming occurs.
Dr. J.
Allan Hobson, a psychiatrist and sleep researcher, believes that since we
always wake after a period of REM sleep, dreaming is a way for the brain to
“warm-up”. In dreams we anticipate the emotions, sights and sounds we’ll
experience upon waking up. In this sense, dreams prepare our bodies to return
to consciousness.7
Another
theory comes from Carl Jung, who believed that dreams are meant to recompense
for the parts of our personality that are less developed when we are awake.
However, a contradicting theory is claimed by Calvin Hall, who studied 2 week
journals from test subjects. Hall states that dreams are continuous with ideas
and behavior when we are awake.8 Even still, there are more opinions
and arguments over what dreams are meant to do. A Nobel laureate named Francis
Crick believed dreams were a way for the brain to discard bits and pieces of
memories that were deemed irrelevant. Crick theorized that dreams were the
accumulation of excess thoughts and ideas that didn’t make it into the brain’s
memory.9 Thus, the reason why we hardly ever remember our dreams.
To this
day, scientists and experts still debate over the meaning of dreams, but across
the controversy, all sides of the debate can agree that everyone has dreams –
although most people forget them by the end of the night.
At a
glance, the mechanics of sleep seem simple: four stages of sleep and dreaming
in between. However, with a closer look sleep is intricately complex and much
more than meets the eye. This time that we spend asleep is not just our body lying
around doing nothing - our bodies use the same amount of energy during asleep
as consciousness, and sleep is just as complicated as being awake. Even today,
extensive knowledge on sleep is not known, but every day experts learn more and
more.
Bibliography
1. Russo, Michael. "Sleep: Understanding the Basics
Causes, Symptoms, Treatment - Stages of Sleep." EMedicineHealth. Web. 14 Jan. 2015.
2. "What Happens
When You Sleep?" National Sleep Foundation. Web. 8 Dec. 2014.
3. Dement, W., and N. Kleitman. "Cyclic Variations In
EEG During Sleep And Their Relation To Eye Movements,
Body Motility, And Dreaming." Electroencephalography and Clinical Neurophysiology 9.4 (2003): 673-90. ScienceDirect.
Web. 14 Jan. 2015.
4. Iliff, Jeffrey, Minghuan Wang, et al. "A
Paravascular Pathway Facilitates CSF Flow Through the Brain Parenchyma and the Clearance of Interstitial
Solutes, Including Amyloid β." National Center for Biotechnology Information. U.S. National
Library of Medicine, 15 Aug. 2012. Web. 14 Jan. 2015.
5. "The Four
Brain States." Tools for Wellness. Web. 14 Jan. 2015.
6. Lüthi, A.
"Sleep Spindles: Where They Come From, What They Do." National
Center for Biotechnology
Information. U.S. National Library of Medicine, 27 Aug. 2013. Web. 14 Jan. 2015.
7. Hobson, J. Allan. "REM Sleep And Dreaming: Towards A
Theory Of Protoconsciousness." Nature Reviews
Neuroscience (2009): 803-13. Nature Review Neuroscience. Web. 14
Jan. 2015.
8. Domhoff, G. William. "The Purpose of Dreams." DreamResearch.net.
Web. 14 Jan. 2015.
9. Breecher, Maury. "The Biology of Dreaming: A
Controversy That Won't Go to Sleep." Columbia University. Web.
14 Jan. 2015.
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