Why We Sleep

The amygdala is the structure of the brain that controls strong emotions, including the flight-or-fight response. Sleep-deprived individuals often act irrational because of the decoupling between the amygdala and the prefrontal cortex. Walker notes that, “we cannot rein in our atavistic impulses—too much emotional gas pedal (amygdala) and not enough regulatory brake (prefrontal cortex)” (147).

Sleeping during two periods over the course of 24 hours characterizes the biphasic sleep pattern. Anthropological, biological, and genetic evidence support that this is our “natural pattern of slumber” (70). Modern society disrupts our ability to follow this sleep pattern. Divorcing humans from this innate practice might be a contributing factor in why our lifespans are shortening.

The circadian rhythm is an internal, natural process that regulates the sleep-wake cycle. Professor Nathaniel Kleitman at the University of Chicago and his research assistant Bruce Richardson discovered that humans, like all other living beings, generate this biological rhythm. In 1938, the pair spent six weeks in Mammoth Cave in Kentucky, one of the deepest caverns on Earth where sunlight does not penetrate its depths. With devices that measured their body temperature and sleep-wake cycle, they made two discoveries. The first was that, even with the absence of sunlight, Kleitman and Richardson continued to generate this circadian rhythm. Thus, humans internally keep time and are not “slave to the sun’s rhythmic commands” (15). The second discovery was that the sleep-wake cycle was not exactly 24 hours. Since Kleitman and Richardson’s experiment, scientists determined the average duration of an adult human’s circadian rhythm is 24 hours and 15 minutes. There are variations in the circadian rhythm across individuals, including morning larks and night owls.

The hippocampus serves as the short-term storage area for memories accumulated during the day. Similar to a USB memory stick, the hippocampus has limited storage capacity. Walker states, “exceed its capacity and you run the risk of not being able to add more information or, equally bad, overwriting one memory with another” (109). The brain handles this memory-capacity challenge during NREM sleep.

The suprachiasmatic nucleus uses melatonin as its “circulating messenger” (22) to communicate the time of day to the brain and body. The rise in melatonin, initiated by the suprachiasmatic nucleus, begins at dusk, signaling to the brain and body that it is getting dark and both should prepare for sleep. As such, melatonin acts as a “biological command for the timing of sleep” (23). Concentrations of melatonin decrease throughout the night. Its absence by the early morning hours informs the brain and body to return to active wakefulness.

One period of sleep within 24 hours characterizes monophasic sleep pattern. Most humans follow this sleep pattern, although it is likely not our innate preference.

The cognitive performance of morning larks is sharpest in the morning. Individuals do not choose to be morning larks. Rather, there is a genetic component to this behavior. Walker argues that the classic nine-to-five work shift unfairly favors individuals who are morning larks.

Narcolepsy is a rare chronic sleep disorder characterized by excessive daytime sleepiness, sleep paralysis, sudden sleep attacks, and cataplexy (loss of muscle control). Scientists believe that alien abduction claims are due to sleep paralysis attacks, which are associated with “feelings of dread and a sense of an intruder being present in the room” (248). These features are similar to how individuals describe encounters with extraterrestrial beings. For individuals who suffer narcoleptic episodes—especially cataplexy—their quality of life can be exceedingly poor because they often need to banish feelings and emotions, which are key triggers of these episodes. Walker notes, “You can well imagine the loss of appetite for such a life” (250).

The cognitive performance of night owls is sharpest in the evening. Similar to morning larks, there is a genetic basis for this behavior. Society’s traditional work schedule does not favor night owls but instead penalizes them by forcing them into an “unnatural sleep-wake rhythm” (21). To maximize the full cognitive potential of night owls, and prevent them from facing health risks associated with sleep deprivation, Walker argues that companies should have flexible work shifts.

Humans generate two different sleep stages, one of which is NREM (non-rapid eye movement) sleep. There are four phases of NREM sleep (stages 1 to 4), with sleep getting progressively deeper with each stage. The eyeballs do not move during this sleep stage. During NREM sleep, the brainwave activity decelerates and is more synchronous. These deep-sleep brainwaves start in the middle of the frontal lobe and travel to the back of the brain. A sleep spindle, or a burst of brainwave activity that protects the brain from external noises, also occurs during NREM sleep. 

REM (rapid eye movement) sleep is the second sleep stage. During this sleep stage, the “eyeballs will jag, with urgency, left-to-right, left-to-right, and so on” (55). However, the eyes are not sending visual information to the brain. The brainwave activity of REM sleep is similar to the brainwave activity while awake, meaning the brainwaves are fast and have no discernible pattern. The key difference between REM-sleep and the awake state is that the voluntary muscle paralysis characterizes the former.