No matter your age, gender, size or shape, everyone on the planet sleeps. Likewise, the consequences of failing to get enough sleep are well-known and well-understood; who hasn’t felt sluggish and lethargic after getting a poor night’s sleep? Since sleep is an integral part of life, it may come as a surprise that scientists are still not completely sure why we need it. Various theories have been proposed over the last several decades, with a new possible answer emerging in October 2013 – it could be that sleep helps to purge the brain of waste.
The Most Common Theories
The most frequent explanations of why humans need sleep often involve repairing the body or maintaining the brain’s ability to function normally. Some of the more widespread theories (and their rationale) are listed below:
Repair and Restoration Theory – The human body can suffer a fair amount of wear and tear throughout the day. The repair and restoration theory holds that sleep allows the body to perform routine maintenance on itself. There is some published research supporting this viewpoint; studies have found that animals who are completely prevented from sleeping suffer from rapidly deteriorating immune systems. Since these animals can no longer defend themselves against harmful bacteria and viruses, they typically die within a few weeks. While humans are asleep, our bodies repair much of the damage to our muscles and tissues, construct new proteins, divide cells and release certain hormones.
Evolutionary Theory – Also known as the adaptive theory of sleep, this theory contends that all species developed sleeping patterns in order to properly utilize energy while they were awake. According to this explanation, humans that generally slept during the overnight hours were less likely to fall victim to predators or suffer from harmful accidents. As a result, humans steadily developed a pattern of sleeping at night and staying awake during the day.
Energy Conservation Theory – While our bodies are sleeping, they use less a good deal less energy. For example, both our body temperatures and calorie consumption decrease while we’re getting some shut-eye. In modern developed nations, where finding food to eat is relatively easy for most people, conserving the body’s energy reserves is rarely crucial for survival. During early human history, however, crucial resources were much scarcer. Given this fact, the energy conservation theory argues that humans who reduced their energy consumption via sleep were more likely to pass on their genes than those that didn’t.
Information Consolidation Theory – Throughout the day, the brain is constantly handling incoming information. An example of such data would include this article you’re currently reading. Sleeping during the nighttime hours, in the view of the information consolidation theory, allows the brain to create long-term memories from all of this fresh info. As an added benefit, a sufficient amount of sleep readies the body for the upcoming day.
A Method for Removing Clutter?
In October 2013, a new idea emerged for why humans need regular sleep, courtesy of a team of scientists from the University of Rochester Medical Center (URMC). This report found that the brain removes unneeded cells while the body sleeps. The UR researchers drew this conclusion after injecting the brains of mice with a type of protein known as beta-amyloid. Beta-amyloid proteins are believed to increase the size of plaque clusters within the brain, which in turn can cause Alzheimer’s and other forms of dementia. Mice were chosen as test subjects because their brains closely resemble those of humans.
During the study, the URMC scientists observed that the mice got rid of the beta-amyloid at a much quicker rate while they slept. Specifically, the mice’s brains removed these injected proteins ten times faster when sleeping than while awake. The report credited this accelerated cleanup rate to the mice’s glymphatic system, which is tasked with removing extraneous materials between the brain’s glial cells (glial cells provide nutrition and protection to neurons, a more well-known type of cell within the brain).
The glymphatic system accomplishes this task by flushing cerebrospinal fluid between glial cells, pushing waste out of the brain and into the circulatory system. Eventually, this waste is expelled from the body entirely. Interestingly enough, these same researchers had earlier found that humans have their own glymphatic system, announcing this finding in August 2012 press release.
Aside from being more active, the URMC study found that the mice’s gylmphatic system was also much more efficient during sleep, clearing out significantly more beta-amyloids. When the mice were unconscious, their brain cells shrank considerably in size, providing the cerebrospinal fluid with a far greater amount of operating space. Researchers noted that the gaps between these cells increased by up to 60 percent while the mice slept.
The study’s lead researcher, Dr. Maiken Nedergaard, noted that dogs and baboons possess comparable cleansing systems, and that it is reasonable to speculate that the human brain uses similar methods to rid itself of toxins. Though the UR study suggests that such a conclusion is certainly possible, Nedergaard hopes to further test her hypothesis with human volunteers at a later date.
Regardless of what your personal sleeping habits might be, the body’s need for sleep will force you to hit the hay at some point or another. Since the exact reasons as to why humans need sleep have yet to be identified, it is safe to say that medical researchers will continue to study this issue in the future.