Why We Sleep: An Underappreciated Mechanism

The Anatomy of Sleep

In order to understand the importance of why we sleep, we must first explain the biology behind this process. In the case of humans, scientists have broken down the process of sleep into 5 distinct stages. The first four stages of sleep are classified as non-REM sleep, with each consecutive stage increasing in sleep depth. The final stage is commonly known as REM, or rapid eye movement, sleep. These two distinct categories of sleep have very different functions in the brain. 

A major function of sleep is consolidation of memories. During the day, memories are stored in the short term memory region known as the hippocampus, similar to a flash drive. When we sleep, the important memories are transferred to long term storage in the cortex. 

Non-REM Sleep: Body and Brain Waves

In the Brain

In the book, Why We Sleep by Matthew Walker, non-REM sleep is compared to the initial process of creating a sculpture. In both cases, large chunks of information are getting “chunked out”. Just as removal of stone is required to create a sculpture, large swaths of memories are removed in non-REM sleep, since most of our daily memories are not required long term. This process decides which memories to keep by preserving memories that were generated with a significant emotional response. 

This makes sense since emotional responses in prehistory meant something was good (and should be repeated) or bad (and should be avoided). We carry this evolutionary quality with us today. In addition, memories learned with the intent to memorize are saved efficiently as well. During these deep stages of sleep, our brains remove the remainder of experiences from your hippocampus to make space for the next day’s ordeal. 

Sleep Spindles

Physiologically, during nREM sleep, our brain send synchronized brain waves from the front of the brain to the back. This is very much unlike our daytime (and REM) brain function, where each neuron is communicating at a different frequency like a crowd filled with chaotic conversation. The synchronized waves allow disruptions in the pattern to travel farther and carry more information. As expected, the brain generates small, controlled, non-synchronized waves called sleep spindles in nREM sleep. These waves act like radio waves; they send information from the hippocampus to the cortex for storage and rational processing.

In the Body

In the body, biochemical processes are occurring. Most well known is the large release in growth hormone, which is amplified by exposure to periods of famine. This hormone signals the body to repair damage that has build up over the waking day. It is important to note that the single most damaging thing our body is exposed to is wakefulness. Both brainwave chaos and bodily micro-trauma generate huge amounts of stress within an organism. From the perspective of our bodies, the reason why we sleep is to recover from the trauma of wakefulness.

Brain Repair

In addition to bodily regeneration, our brains have a macroscopic problem that can also explain why we sleep: sewage. During the day, toxic metabolic products build up between our brain cells. These chemicals cannot get drained into the vessels due to the tightness of the blood brain barrier. As a result, our bodies use sleep in a fascinating way. When we go to sleep, the cells surrounding our neurons shrink. This allows the fluid from our spine to seep between the brain tissue and wash away all of the piled up junk. And what is the largest chemical that this process clears out? None other than the stars of Alzheimer’s: Beta Amyloids! 

REM Sleep: Dreams and Memories

After nREM sleep does its work, REM sleep takes over. In terms of brain activity, our brains appear to have the same exact wave patterns that we have when awake. This means that for all intents and purposes, we are awake during REM. The only difference in our bodies during the dream state is that our brains paralyze our bodies. This makes sense because during dreaming, our unconscious mind hijacks the sense regions of the brain so we can experience a dream. If we were not paralyzed, our movements in the dream would be accompanied by the unconscious motion of our bodies in reality. This is what malfunctions in sleepwalkers. 

During this stage of sleep, our brains are trying to figure out how the newly selected memories from the past day fit into all the older experiences we have remembered. As a result, our minds become a sandbox, with old memories clashing into new ones as we figure out how different memories fit together. As you can imagine, this state of biological creativity may also be useful for coming up with creative solutions to complex problems, which may explain the evolutionary significance of why we sleep. The best example of this phenomenon is in the mind of Dmitri Mendeleev, who discovered the periodic table in his sleep after many failed wakeful attempts.

In addition, dreams provide the unique ability to dull the emotional response to a memory. Think of the scariest or most painful experience you have ever had. I can guarantee that the memory of the experience you just recalled was not nearly as scary or as painful as the real experience. This is why you didn’t just scream in fear (or pain) when recalling the memory. During dream sleep, the stress hormone nor-adrenaline is shut off, allowing the brain to dull emotional trauma during memory formation. In the case of PTSD, the dream state is unable to shut off this hormone, resulting in the victim reliving the experience every time they remember it.