Chapter 8: Circadian Rhythms and Sleep
8.3: Why do we sleep?
All organisms that we know of experience some type of sleep. But we still haven’t figured out exactly why animals sleep. Here, we will discuss three theories that have been proposed to explain sleep. None of these theories alone fully explains the complex phenomenon of sleep, and they are not mutually exclusive. The most likely reason we sleep is probably some combination of the following three theories.
Case study: Peter Tripp
In 1959, New York City radio DJ Peter Tripp ran a publicity stunt to raise money for the charity March of Dimes: he stayed awake for 201 hours. Sitting inside a glass booth in the middle of Times Square, Peter played music and broadcasted his experiences across the airwaves. The first night of sleep deprivation wasn’t awful, but the next seven consecutive days and nights were a real challenge, for both Tripp and the doctors who kept an eye on him.
Just a few days in, Peter began experiencing severe psychological side effects. Tripp developed intense paranoia: He mistook his psychiatrist, who was wearing all black, for being the undertaker here to collect Peter’s body for a funeral. Tripp even began hallucinating, seeing spiders and rodents in his clothes – prompting him to strip naked and run into the street screaming. Some believe the long-term sleep deprivation severely affected his brain beyond this temporary psychosis. After the experiment, Tripp was involved in a huge commercial bribery scandal, lost his job, and got divorced (Of course, life has many variables, so we’ll never be completely sure of the precise effect long-term sleep deprivation may have had on his relationships and decisions.) Today, the Guinness Book of World Records no longer allows people to compete for the prolonged- wakefulness record due to health concerns.
Recuperation Theory
The recuperation theory of sleep is centered around the idea that being awake is stressful and exerts a physically demanding toll on the body. The body therefore needs a period of time when energy usage decreases and the body’s natural repair systems can work without disruption. Sleep is how the body “wipes the slate clean” and resets.
Evidence for the recuperation theory comes from experiments tailored around the idea of looking at what happens when a person doesn’t get sufficient sleep. As anyone who has ever pulled an all-nighter can attest, a single night of sleep deprivation often leads to significant psychological changes, including anxiety, irritability, and mood swings. Staying awake even longer than 24 hours can cause more severe changes in mind state, such as temporary psychosis, hallucinations, or delusions.
The recuperation theory is supported by several pieces of evidence, three of which we will discuss.
- Enhanced metabolic cleaning during sleep. When you’re awake, your cells produce several biological waste products during cellular respiration. These chemical byproducts can be potentially toxic to the body when they accumulate. The glymphatic system, a sort of cellular rinse that floods the extracellular space with CSF, clears these cellular waste byproducts when they accumulate in the brain. When we sleep, the extracellular space expands by about 60%, which increases the ability for CSF to penetrate deeper into the brain tissue. One byproduct of interest is the molecule beta-amyloid, a protein that exists in the healthy brain. But, accumulations of beta-amyloid are found at high levels in the post-mortem analysis of brains from patients with Alzheimer’s disease. During sleep, the glymphatic system’s “rinsing” activity washes away beta-amyloid from the interstitial space and degrades the protein.
WAKE SLEEP
Figure 8.5 During sleep, the glymphatic system increases flow of CSF (green waves) that are able to wash out the amyloid-beta protein (black dots) from the interstitial space. Modified from https://commons.wikimedia.org/wiki/File:Video_schematic_of_glymphatic_flow.ogv
- 2. Immune system function improved with sleep. Sleeping more enhances your ability to fight pathogens. Sleeping for fewer than 6 hours per night increases the likelihood of catching a variety of transmissible illnesses, such as cold, flu, or gastroenteritis. Sleep increases the effectiveness of vaccines, and each hour of sleep over 6 hours increases said effectiveness by about 50%.
- 3. Increased production of growth hormone during deep sleep.Most cells of the body are turned over regularly as cells die and get replaced. One of the signaling molecules that encourages the replacement process is growth hormone (GH), which enhances cellular repair, muscle and bone growth and protein synthesis. Normally, throughout a 24-hour day, GH is produced and released throughout the body by the hypothalamus. The largest wave of GH production occurs early in sleep, during NREM 3 sleep. During this huge burst of GH release, a person’s circulating plasma GH concentration may be 10 times higher than at baseline.
While the support for the recuperation theory is generally true for almost all people, there are about 1% of people who seemingly gain the restorative benefits of sleep, even with fewer than 6 hours of sleep each night. They may wake up at 4:30 in the morning feeling completely refreshed. And yet, despite getting so little sleep, these short sleepers have similar health outcomes with respect to body mass index and psychiatric measures such as depression and overall optimism. Something about the circadian rhythms of these short sleepers allows them to “maximize” their sleep efficiency. Many have very short sleep latencies, meaning they fall asleep within minutes after lying down – as quickly as someone with narcolepsy. They also spend a larger percentage of their night in deep sleep and REM sleep while minimizing NREM1 and NREM2.
Figure 8.6 Growth hormone levels peak early in the night during deep sleep.
Evolutionary Adaptation Theory
The evolutionary adaptation theory is the idea that animal sleep patterns are different across species for reasons that most efficiently benefit each animal. Over millions of years of evolution, individuals with the most ideal sleep patterns have an advantage, and their sleep habits will be selected for in the following generation.
For example, consider humans. As an animal highly dependent on light and the visual system for navigation and accurate performance of tasks, the dark is a very dangerous time to be active. The risks of wandering off a cliff, running head-first into a tree while escaping a predator, or eating a wrong-colored poisonous berry would all be elevated in the dark. We benefit from behaviors that minimize those risks, such as inactivity until the sun rises. During this inactive period, sleeping decreases our metabolism and our body’s need for energy.
Figure 8.7 In a traveling pod of dolphins, the animals on the edges sleep with half of their brain in order to remain a vigilant lookout. https://pixabay.com/photos/dolphins-fish-mammals-delphinidae-380034/
Humans are just one animal that has an evolutionary fine-tuned sleep pattern. If you look across the animal kingdom, you’ll find all varieties of sleep behaviors that are best fitted to the needs of the individual species. In the wild, for example, dolphins are generally prey. They evolved with the ability to put one half of their brain to “sleep” at a time, allowing the “awake” half to keep an eye out for potential predators. Small prey animals, like squirrels, are faced with the threat of being attacked at night. For them, remaining very still, quiet, and hidden improves their survival. Tigers, the top alpha predators in any ecological niche, have almost no predators to hide from, allowing them the luxury of sleeping up to 20 hours a day.
This evolutionary adaptation theory argument has a major weakness, however. In almost all animals, sleep represents a period of time when an organism’s ability to use their sensory organs to detect the hallmark signs of an approaching predator, like the flurry of feathers from a hungry owl or the soft padding of a wolf footsteps, decreases drastically. For an animal that can’t hide very effectively, sleep represents a period of vulnerability, as they would be unable to sense incoming threats.
Figure 8.8 Alpha predators have no fear of being attacked, and are given the luxury to sleep for prolonged periods of time. https://pixabay.com/photos/tiger-big-cat-majestic-noble-zoo-3383616/
Brain Plasticity Theory
The brain plasticity theory suggests that the brain needs some period of time for critical changes to occur. During sleep, circuits in the brain undergo consolidation processes that are important for memory formation. For example, academic performance and examination grades worsen as a person’s nightly sleep decreases.
The evidence in support of this theory starts with looking at the brains of newborns. When you were first born, during those first few weeks of life, you slept close to nearly 70% of the day, almost 17 hours! At this point in your life, your brain starts to experience all sorts of new sensations: your eyes detect visible light for the first time, the skin feels the air blow past it, and the ears sense new frequencies and combinations of sound waves. As a result of these stimuli, scientists hypothesize that your brain undergoes as many learning events as rapidly as possible. This rapid learning helps you remember what you learned each day so you can respond to your environment as you grow up.
Figure 8.9 According to the brain plasticity theory of sleep, newborns spend most of their day sleeping in order for their brains to adapt to all the new senses they are experiencing. https://pixabay.com/photos/father-baby-portrait-infant-22194/
