Why do we sleep?
    Do you realize just how much time is spent sleeping? The "average"
human requires about 8-hours of sleep every day, which amounts to 56
hours per week, 240 hours per month and 2,920 hours per year! Indeed,
one third of our lives is spent in a state of relative unconsciousness
and inactivity! Everyone (especially the college students) has
occasionally wondered why we might need such a thing as sleep. Yes,
North American college students must hate sleep. They don't go to bed
until 3 or 4 a.m. in the morning thinking that sleeping is a complete
waste of time that could be better spent partying or watching TV (or
perhaps studying...I guess), but is sleep really nothing but a needless
distraction? Even though you may want to spend less time on sleep, you
still understand that sleep is necessary for your survival.
Furthermore, though it appears that the body enters a relatively
motionless and seemingly unconscious state during sleep, the brain is
actually very active. This seems to be somewhat counter-intuitive on
the surface.
    What is "sleep" after all? From a scientific view, it is not
sufficient to define sleep as a lack of consciousness, since it is
clear from brain activity patterns that being unconscious is different
from being asleep. A common way scientists record brain activity is by
attaching electrodes to the scalp of a subject. The electrodes are
then connected to a machine called an "electroencephalograph", which
produces the wavy lines of an electroencephalogram (EEG) to illustrate
what most people know as "brain waves". In the modern age, sleep is
defined in the context of EEG patterns and from numerous studies,
scientists have found that, contrary to common belief, sleep is not a
single, uniform state but instead can be divided into two distinct
phases: rapid-eye-movement (REM) sleep and non-REM (NREM) sleep.
Human adults generally complete the two stages in 90 minutes or 4-5 times
during an 8-hour period of sleep.
    During NREM sleep, the EEG is dominated by high-amplitude and low-
frequency synchronous waves. This phase can be further divided into
four sub-stages (Stage 1, Stage 2, Stage 3 and Stage 4) according to
their EEG patterns. Stages 3 and 4 are sometimes called slow wave
sleep (SWS), during which the frequency of the brain waves is less
than 2 Hz. Low EEG frequencies do not necessarily mean that the
neurons in the sleeping brain fire less, but rather, that neurons are
firing in a highly synchronized fashion. REM sleep is a relatively
deep stage of sleep and is characterized by the presence of rapid eye
movements as its name indicates. During this stage, low-voltage and
high-frequency waves become prominent in EEG recordings. Surprisingly,
the REM EEG looks almost identical to waking EEG patterns. Most dream
reports have come from awakenings during this stage. The REM sleep
stage has consequently been associated with dream production for a
long time. The story is not quite that simple, however, since dreaming
occurs during NREM sleep to a lesser degree and are not easily
recalled. Interestingly, during REM sleep, the muscles of the entire
body expect those controlling eye movement and respiratory system are
completely paralyzed, which prevents us from acting out our dreams.
There are, unfortunately, some degenerative brain diseases in the
elderly that can cause aberrant REM sleep behavior disorders. Due to
their inability to inhibit the expression of motor actions in REM
sleep, these people may literally act out their dream scenarios, which
may lead to tragic outcomes. In summary, awakeness, NREM sleep and REM
sleep are the three distinct states of brain function accompanied by
large shifts in body functions.
    But why sleep at all? The concise answer is...nobody really knows,
although scientists do have quite a few "theories". The 3 basic
theories are: 1) sleep is an adaptive process, 2) sleep has a
restorative function and 3) sleep plays a role in learning and memory
consolidation. According to the first theory, some have argued that we
do not really require sleep at all. It is an inherited adaptive
behavior that was a result of evolution. Sleep may have developed
because of a need of animals to protect themselves. For example, some
animals search for food and water during the day because it is easier
to see when the sun is out. When it gets dark, it is best for these
animals to save energy to avoid getting eaten, and possibly falling
off a cliff that they cannot see. When comparing sleeping habits,
animals that serve as prey for predators sleep the least. Conversely,
the adaptations linked to animal order appear to have relatively
little to do with determining the length of REM sleep, as the REM
sleep time can vary greatly even across close species. Sleep surely
has many adaptive functions like conservation of energy, repair of
injury, and defense from predation. But is that all?
    The majority of scientists believe that sleep is a restorative
process. That probably makes perfect sense to most people. You go to
bed because you feel fatigue, and you feel fresh again after having a
good sleep. The restoration theory suggests that sleep helps the body
recover from all the work it did while the individual was awake.
Experiments have shown that the more physical exercise an animal does,
the more NREM the animal will have (but this is not true if comparing
between different species?. Moreover, if people are deprived of NREM
sleep by being awakened every time they get to stage 4 SWS, the
majority complain of being physically tired. So, it appears that sleep
not only conserves energy, but also restores energy. In fact, a group
of researchers have shown that NREM sleep restores brain glycogen
stores. A chemical called adenosine is also believed to play an
important role in sleep. But in contrast to the energy molecules such
as glycogen that is being restored during sleep, our brain is trying
to get rid of exceeding amount of adenosine during sleep. The role of
adenosine became clear when scientists tried to figure out how
caffeine works to keep us stay alert. Researchers once believed that
the chemical blocked the same sedating mechanisms that tranquilizers
induced but soon scientists found that caffeine actually blocked the
receptors (i.e. the special receiving areas on cells) of adenosine and
thus prevented its effect. Now, increasing evidence is confirming that
adenosine is indeed an important factor that induces fatigue and
sleepiness. Adenosine originally comes from a large energy molecule
known as adenosine triphosphate (ATP). As ATP loses chemical groups
known as phosphates, energy is released from the molecule to fuel
billions of other reactions that take place in the cells when we are
awake. Finally, ATP is dwindled down to adenosine, which then gets
accumulated in the brain as a result of prolonged wakefulness.
However, we don't know yet how the brain reduces the concentration of
adenosine during sleep. Some researchers are now attempting to
determine whether sleep relates backs to ATP by examining the
relationship between adenosine and ATP on a cellular level.
    Recent findings have indicated that adaptation and restoration may not
be the sole purposes of sleep. Surprisingly, an increasing amount of
research in both animals and humans has suggested that sleep also aids
in securing memories and helps learning. Actually, the idea that sleep
is important for learning and memory has been hotly debated for years.
Recent advances in neuroscience research have allowed scientists to
conduct a variety of careful studies in both animals and humans, which
have provided clear evidence that sleep is indeed important for at
least some types of memory and learning. Now it is believed that REM
sleep is especially important for procedural learning (i.e. the
learning of a motor skill or action performance such as riding a
bicycle or playing a melody on the piano), whereas slow wave sleep has
been shown to be of greater importance in the consolidation of
explicit or declarative memory, which involves recall of specific
events or knowledge (such as the memory of what you've eaten for
today's lunch or knowing who's the most stupid US president). It has
also been reported that both REM and SWS are required for the
consolidation and processing of emotional memories.
    Testing the hypothesis that sleep is required for memory is not a
straightforward task. Some sleep-memory studies have relied on a sleep
deprivation paradigm, which obviously had a great number of secondary
effects on attention, disposition and motivation that may have
complicated data interpretation. Nevertheless, there are a number of
intriguing observations consistent with a role of sleep in memory
consolidation. For example, in one human study, participants had to
perform the simple motor task of repeatedly typing a sequence on a
keyboard. The results showed that a full night's sleep after training
improved their performance by almost 20 percent, compared to the
performance before sleep. Another study also has shown that sleep is
required immediately following learning since participants who were
sleep deprived the night following training but were allowed two
subsequent nights of recovery sleep showed no significant task
improvement.
    How does sleep aid learning and memory? Sleep may allow the brain to
reprocess newly learned information so that memories can stick longer.
This idea is supported by the results from the studies using a
technology know as brain imaging to map brain activity. Several
research groups have found that the brain areas that were activated
during procedural task training were re-activated during REM sleep.
Furthermore, during REM sleep, the brain areas that showed significant
activation during task performance were more active in trained
subjects than in untrained subjects. In other words, it appears that
our brain "replays" learned experiences while asleep, probably as
part of a process of memory consolidation. In addition, there are a
number of correlative studies suggesting that the loss of these of
"replay" episodes could lead to memory deficits.    
    As per usual, reality is rarely as simple as theory. There are reports
from literature that are against the idea of the necessity of sleep
per se in memory consolidation. For example, patients with specific
types of brain injury or on certain types of medication can have
profound disruptions in sleep patterns or do not sleep at all. There
are also a few examples of individuals who spontaneously become
insomniac for their entire remaining lifetime. But these phenomena do
not lead to any profound memory disruption in these individuals,
apparently dissociating sleep from memory formation. Given the
ambiguity in defining sleep in these reports, however, it is certainly
possible that these insomniac individuals may still have certain sleep-
like patterns of EEG activity that may serve as functional substitutes
of sleep. For the time being, no one really knows.
   Obviously, more additional studies are required to clarify the role of sleep
in learning and memory consolidation. At this point, however,
the evidence provides enough arguments that it's time for
undergraduates to stop staying up all night and catch some zzzz's,
especially before exams. And for those who need an excuse for getting
some extra sleep, just say, "I'm not sleeping; I'm learning."
 
K.Fan