Hiu Yu Wong
Try explaining the state of sleeping to an alien. “We spend a third of our lives lying down without perceiving the environment around us, and sometimes there’ll be imaginary stories happening in our head…” Indeed, the act of sleeping sounds absurd and impractical. Why do we waste our time on our bed when we could be working, or reading a book instead? Sleep seems to hinder our productivity and take up precious time. Many of us therefore choose to stay up late, working on homework or studying, or just to watch more episodes of a beloved TV show. However, sleep serves various functions and has numerous health benefits, as we will discover.
The urge to sleep comes as a result of two factors — our internal circadian rhythm and the release of adenosine. Our circadian rhythm acts as an internal 24 hour clock, and when night comes, a structure called the suprachiasmatic nucleus in the brain (just above the intersection of the optic nerves) causes the pineal gland to release a hormone called melatonin, signalling to the body that it is night. Adenosine also accumulates in the brain throughout the day, creating a pressure to sleep by turning up the sleep-inducing regions of the brain. As we begin to sleep, the sensory gate in the thalamus becomes blocked, and we stop perceiving our external environment.
As we slip into a slumber, there are 2 stages of sleep that we cycle through every night: non-rapid eye movement (NREM) sleep, and rapid eye movement (REM) sleep. Brainwave activity in NREM sleep is slow and synchronous with high amplitude and low frequency, whereas in REM sleep the brainwave activity is frantic and erratic, resembling that of waking brainwave activity. It is in REM sleep where dreaming occurs.
Why, then, do we sleep? Without sleep, we get tired the next day, or become easily irritated. With a good night’s sleep however, we wake up feeling refreshed and restored, ready for the day’s events. What is it that requires 8 hours of shut-eye for our normal functioning?
You may think lying down for hours on end seems quite useless, but it is quite the contrary. Sleep offers multiple benefits — it consolidates memories, for a start. At night during NREM sleep, memories are transferred from the hippocampus to the cortex, from short-term to long-term. This frees up the hippocampus and allows new memories to be made the next day. The transfer of memories helps consolidate the memories from the previous day while regaining capacity for short-term storage for the next day’s learning. It has also been shown that better REM sleep quality (induced by suppressing noradrenaline in the brain) decreased frequency of nightmares in PTSD patients. I’m sure everyone’s gone through a situation where a challenging maths problem was resolved quickly after a night’s sleep — that is one of the benefits of dreaming in REM sleep, where creativity is inspired. Mendeleev dreamt of organising the elements by atomic number and created the periodic table, and neuroscientist Otto Loewi’s Nobel Prize-winning experiment on communication between neurons came to him in a dream as well. Sleeping and dreaming build the connection between different ideas and concepts, allowing problems to be solved.
Aside from benefits to the brain, sleep also reduces the risk of cardiovascular diseases, controls and maintains a healthy appetite, helps with reproductive fitness and leads to a stronger immune system.
As for the fundamental function of sleep, there are numerous theories, from energy conservation to cellular restoration. One in particular is the brain plasticity theory. Synaptic homeostasis hypothesis (SHY) proposes that sleep’s rudimentary function is to restore synaptic homeostasis. The brain is plastic, and allows changing and adapting in synapses. During waking, our brain is burdened with multiple tasks, and plastic changes in the brain are biased towards potentiation, including increase in the number of synapses and synaptic strengthening. However this condition cannot be sustained — it requires higher consumption of energy, need for cellular supplies like glucose, and occupies more space. Therefore synaptic strength must be regulated and returned to a suitable level. Costs of energy, space, supplies could be restored as well. This synaptic homeostasis would be achieved during sleep where there is no demand for learning.
Another theory is the restorative function sleep serves in the brain. In the brain, the glymphatic system controls the flow of cerebrospinal fluid (CSF), a clear liquid that surrounds the brain and the spinal cord. Brain cells called glia controlled the flow by shrinking or swelling. Glial cells of the brain shrink by up to 60% during NREM sleep, enlarging the gaps around the neurons, allowing cerebrospinal fluid to clear out metabolic waste. This was also studied in the context of Alzheimer’s disease, where a toxic protein beta-amyloid builds up in plaques and destroys brain tissue. It was observed that beta-amyloid disappeared faster in mice’s brains when mice were asleep, suggesting that sleep clears toxic molecules in the brain. The glymphatic system in the brain removes harmful metabolic waste from the neurons during sleep, and thus sleep would be crucial.
These theories provide a picture of the functions of sleep. Although a precise reason has not been identified, it is undeniable that sleep fulfills multiple physiological purposes and brings about numerous health benefits. This inescapable, inevitable part of our life has many mysteries to offer, and there is still much to explore in the topic of sleep and neuroscience. Perhaps we will better appreciate the enigma of sleep by succumbing to it. To sleep or not to sleep? You can choose the answer.
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