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The 20th-century evolutionary biologist, Theodosius Dobzhansky, said, “nothing in biology makes sense except in the light of evolution.” From the perspective of evolution and its twin fitness functions of survival and reproduction, sleep is a seemingly difficult phenomenon to make sense of and explain. When we are asleep, we cannot gather food, socialize, find a mate, or protect ourselves or our offspring against predation. Then why is it we sleep? Does it serve an adaptive function or is it merely an epiphenomenon? An observation that supports the hypothesis of sleep serving an adaptive function is that it is not only specific to humans. In fact, every species that has been studied sleep. Amongst mammals, elephants sleep for four hours, tigers and lions for fifteen, and humans for eight. Therefore, it can be considered that sleep confers a substantial fitness advantage. Sleep has been shown to enhance learning, decision making, facilitate emotional regulation, boost immune function, regulate appetite, and modulate our cardiovascular system. In fact, there does not seem to be one major organ system that is not enhanced by sleep. It can be considered an evolutionarily selected suite of mechanisms for bodily repair and restoration.

Sleep is not just an absence of wakefulness, but a complex, metabolically active, and deliberately choreographed series of unique stages. In humans, sleep progresses in a series of four or five regular dreamless non-rapid eye movements (NREM) and dreamful rapid eye movement (REM) throughout the night. The brain cycles through various stages of sleep approximately every 90 minutes with REM episodes generally lengthening as the night goes on so that by the early morning hours we are predominantly in REM sleep. Humans spend between 20-25% of sleep in REM sleep and the remainder in NREM sleep. Interestingly, NREM and REM sleep are as different from each other as they are from wakefulness. Upon first falling asleep, we fall into stage NREM sleep and progress towards deep NREM (stages 3 & 4). The deep sleep stage is characterized by the delta wave on an electroencephalography (EEG) readout. The brain becomes less responsive to external stimuli and a person is most difficult to arouse. Physiologically there is an overall decrease in metabolic rate as compared to wakefulness. REM sleep, on the other hand, occurs approximately 90 minutes after the onset of sleep. The characteristic outward feature of REM sleep is the rapid side to side eye movement. More importantly, although we are temporarily paralyzed, regions of the brain associated with emotion (amygdala, anterior cingulate), memory (hippocampus), and processing visual information are selectively activated and regions of the brain associated with rational thought and logical decision making (prefrontal cortex) are deactivated. Not surprisingly, one third to one-half of dreams have emotional themes.

During NREM sleep, the sympathetic nervous system (fight or flight) is relatively deactivated. This enables parasympathetic mediated restorative functions of hormones such as the growth hormone to take effect. Growth hormones facilitate the repair of blood vessels, stimulates division and multiplication of chondrocytes of cartilage and also stimulates the immune system. Within the brain, NREM sleep has been shown to enhance learning, retaining learned information, and consolidate the gains of practice via improved skill (muscle) memory. Similarly, despite the increase in metabolic rate and the overriding emotional content of dreams during REM sleep, the brain is devoid of norepinephrine (fight or flight hormone) activity. It is hypothesized that this is important as it provides a “stress-free” environment to recalibrate and fine-tune the emotional circuits of the brain. REM sleep and the dreams it contains are key for emotional regulation and interpreting social cues. The latter might be one of the defining features of the human brain. Lastly, although NREM sleep strengthens individual memories, it is REM sleep that offers the benefit of fusing and blending those elemental ingredients together in abstract and novel ways.   

Aldous Huxley wrote, “that we are not much sicker and much madder than we are is due exclusively to that most blessed and blessing of all natural graces, sleep.” What he knew through experience and observation is being experimentally tested and validated via the scientific method. We often talk about exercise and nutrition as foundational elements of a healthy lifestyle. However, it is probably not an exaggeration to include sleep as the third pillar of that foundation. Individually, and as a society, we under-appreciate the restorative functions of sleep. Evolutionarily speaking, sleep must have a substantial fitness advantage considering that we spend one-third of our lives in a helpless, sex-less, and food-less state. For many people in the industrialized world, shift work is a necessity. In my next essay, I will explore the disruption of the circadian rhythm and its impact on shift worker health, decision making, and emotional regulation.

Reference: Why We Sleep