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Since ancient times, humanity has strived to understand sleep. The ancient Egyptians, Greeks and Romans all garnered different beliefs as to what sleep meant and even named gods who brought dreams to slumberers. In fact, up until the 1950s, sleep was thought to be a time of passive activity in which both the mind and body lay dormant. Today, however, we know that is not the case. Various mental activities occur during sleep, and the phenomenon provides many functions we need to survive. While the subject still has an aura of mystery, we can delve into what we currently know about the science of sleep.
Stages of Sleep
Sleep is known to cycle through two phases, non-rapid eye movement (NREM) or quiet sleep, and rapid eye movement (REM) or active sleep. Non-REM sleep is split into three stages, each of different intensity. Stage 1 marks the transition between wakefulness and sleep. Although it only lasts 5-10 minutes, it is associated with the slowing of heart rate and breathing, as well as the relaxation of muscles and brain waves. This stage is also the time in which you can be awakened most easily.
Following this, stage 2 is a deeper sleep, which normally means a continued slowing in heart rate and breathing, as well as a decrease in temperature and eye movement. Approximately 50% of all sleep is in this stage, which normally lasts between 30 and 60 minutes. NREM stage 2 also begins to produce bursts of rapid, rhythmic brain wave activity, which are known as “sleep spindles”. These are theorized to be a part of memory consolidation, in which the previous day’s events are filtered into one’s consciousness.
Stage 3 is arguably the most important stage, as it is associated with physical reparation and declarative memory. Declarative memory includes facts, statistics, general knowledge, and other learned information — which is especially important for students. Stage 3 embodies further slowing of heart rate and breathing, and the deepest sleep within the cycle — lasting between 20 and 40 minutes.
Now, REM — or active sleep — begins. This phase is key to the development of dreams, and is one of the most mysterious aspects of sleep science. At the start of this phase, your brain begins high activity and your eyes begin to move rapidly. This is accompanied by full muscular immobilization, and increased breathing rate. Memory consolidation continues in this stage, but this time it is supplemented by dreaming — or paradoxical sleep. Paradoxical sleep first lasts for about 10 minutes, but increases in length each REM phase of the sleep cycle. Since the average person experiences the full cycle 4-5 times a night, one may reach REM sleep 4-5 times as well.
Sleep Controls
Interestingly, the human body is equipped with natural “sleep controls” which dictate when to sleep and for how long it should last. The first of these functions is known as circadian rhythm.
Circadian rhythms are connected to a biological clock, which is especially sensitive to lighting. The bio-clock begins the high production of melatonin — a key hormone that evokes tiredness — when in the dark, and halts this process when exposed to heavy light.
Sleep drive, the second sleep control function, also assists in tiredness. Similarly to how your body hungers for food, sleep drive is the body’s desire for sleep. It increases throughout the day, and normally peaks late at night. However, while hunger is unable to force food consumption, sleep drive is able to instantaneously activate sleep after a long enough time. Sleep drive can be offset by small naps throughout the day, but this could upset one’s natural sleep cycle.
Why We Need Sleep
Sleep represents one-third of our lifetimes, and for good reason. It serves a multitude of beneficial functions, and the lack of sleep is often quite detrimental. Primarily, brain function following a night of sleep deprivation is significantly decreased, especially in the scope of brain plasticity. Brain plasticity is the ability to adapt to input, and a decrease in this function will mean slow response time, poor memorization, and general fogginess of the mind. Additionally, a lack of sleep can increase the risk of seizures, high blood pressure, depression, and migraines, as well as a decrease in metabolism.
Overall, sleep plays a key role in both mental and physical health, and will continue to do so for the full extent of our lives. While certain aspects may still be shrouded in mystery, the importance of sleep and the impact it has on all of us is clear.
