Did you every think about what a huge disadvantage it is to have to basically shut down for hours every day? In the wild, an animal is at the mercy of predators. Further, we and they have to spend significant amounts of time finding or preparing places to sleep. And if you are having food security problems, this preparation and sleep time could have been much better used finding calories. Because mammals and birds burn almost as many calories awake as when asleep (only about a 10% difference).
I imagine that some busy people think of sleep as an annoyance…they could use those extra hours each day more productively. But they are wrong.
A Mystery Thousands of Years of Years Old
People have wondered for a very long time why humans and animals sleep. Theories have abounded. When I was younger, the prevailing theory was that our brain needed to sort through the day’s memories and put some in long-term storage. Before that, people postulated that sleep was an evolutionary adaptation to keep animals out of dangerous situations for part of the day. Still others thought that our bodies needed the rest. Others thought that our brain needs a sleeping time to lock in learning of methods and movements, which they thought explained why infants need so much more sleep than adults.
But none of those theories make sense as a full explanation.
If you don’t sleep for 4-11 days, you will die. The world record for humans going without any sleep is only 11 days. And that person may have dozed off with his eyes open or entered sleep-like states while “awake.” Mice and fruit fly studies show that if we stop those animals from sleeping, they quickly die. Even a small amount of sleep will stave off death, but keeping a person awake for many days leads to neurodegenerative failure and death.
If memory storage were the problem, we would just get memory problems, not die. If sleep only serves to keep us out of danger, there is no evolutionary reason that a lack of it should cause us to die. People with extremely sedentary lifestyles (like blog writers) still need sleep or they die, so it’s not necessitated by a body needing to rest. Lack of sleep does make learning harder, and sleep is correlated to locking in new task abilities. But there is no reason that a lack of being able to learn tasks should make us die.
I remember doing a moderate amount of research on why we sleep back in 2009/2010. I came away thinking that we simply don’t know. I told a number of people back then that it was one of the most important current medical mysteries.
As a side note, it seems like a lot of people, including medical experts, were more comfortable publicly espousing poorly-thought-out theories than saying that “nobody knows.” This is important to note. Look for it in other arenas and avoid it yourself.
The 2013 Breakthrough
In work published in late 2013, Dr. Maiken Nedergaard at the University of Rochester and her team injected fluorescent dye into mouse craniums just under their brains. As one does. The dye saturated the cerebrospinal fluid in which the brain bathes. These researches then watched where that fluid seeped into the mouse brain tissues using advanced real-time imaging techniques.
Another side note: They used something called two-photon imaging to see into the brains. This uses infrared lasers to excite the fluorescent dye molecules. Except normally, the dye molecules won’t get excited by individual infrared photons. But if two infrared photons hit the same dye molecule at exactly the same time (within femtoseconds of each other), it’s enough to add their photon energies together and excite the fluorescent molecule, and then the dye molecule emits a stronger photon. This stronger photon is at a higher, more energetic color (wavelength) than the infrared photons flooding the tissue. For instance, it may be blue light. Special filters can be used to see this emitted blue light while blocking from the imaging camera the flood of laser infrared light. Those blue light detectors can tell exactly where the blue light is coming from, and so can create insane 3D models of great precision and beauty, even of living tissue (the infrared lasers penetrate but don’t harm living tissue). Here is an example of the maps of living neurons that can be created by this technique:
Back to the fluorescent dye injected into the mouse brains: The dyed cerebrospinal fluid didn’t soak very much or very far into the mouse brains while the animals were awake. But when the mice were asleep or anesthetized, the researchers saw that the fluid swept far and fast into the the depths of the mouse brains. It could also easily flow back out of the brain tissue. This fluid was then extracted by the researchers, and they found several types of toxins and waste products produced by brain cells now saturating the cerebrospinal fluid. The fluid had washed the toxins from the deep recesses of the mouse brains.
These brilliant researchers discovered that when animals (and humans in subsequent studies) are awake, the brain is puffed up and more dense in its working, conscious state. But when we let go of consciousness and drift into sleep (or anesthesia), our brain un-puffs, opening up big and also tiny channels that let the cerebrospinal fluid infiltrate deep into all the parts of our brain and wash away toxins.
It turns out that those toxins are the normal byproduct of brain cell operation, but when the brain is in the waking state, they can’t get out of the brain tissue. However, when the cerebrospinal fluid can come into the deep recesses of our brains and wash those toxins out, the fluid is then filtered by specialized lymph nodes in the area which take those toxins to our blood, and then the toxins are filtered out by our liver.
When we don’t sleep enough, those toxins build up. Our brains detect this poisonous build-up, and makes us sleepy so that we can let our brains literally deflate and get washed clean of toxins.
This explains so much. It explains the necessity of sleep. It explains why our brains work worse when we haven’t had enough sleep (our brains are wallowing in toxic waste products!). It explains why we can experience permanent neurodegenerative damage if we are forced to stay awake for 4-11 days. And it explains why forced lack of sleep can kill us.
It also explains why chronic (regular) lack of sleep is so dangerous to our health, especially as we age. Lack of regular, long sleeps is linked to many bad health outcomes, including premature death.
A meta analysis of sleep studies found that people sleeping less than 5 - 7 hours a night have a 12% greater risk of death. The increased risk for older adults could be due to the decrease in REM sleep as we age. Leary et al found that “there was a 13% higher mortality rate over 12.1 years for every 5% reduction in REM sleep”
Another fascinating corollary of this work is that holding our conscious selves is some sort of special and difficult work for our brains. When we are reacting vigorously with the world and exerting our inner voice to navigate reality and solve problems, our brain is in a sort of overdrive. This fully-engaged conscious state requires our brain to be puffed up, perhaps to allow small arteries and capillaries to deliver blood at the highest possible volumes to feed our very active neurons.
But when we don’t need to hold our consciousness quite so tightly, our brains can relax and deflate and let the fluid wash away the poison, cleansing our cerebral organ.
Everything above is hard science, recently discovered but carefully proven out by Dr. Nedergaard, her brilliant team, and now many other researchers. But for just one paragraph I will speculate based on an interesting observation I have made. Take it our leave it:
I have noticed an activity that, after it is done, feels like I have just slept. In fact, I could be quite sleepy to the point of internal head pain just from sleepiness, but after I do this thing I sometimes feel like I just had a rest. It is simply watching a very engaging movie or TV show. What do TV watching and sleep have in common? To me, nothing. It’s silly to even mention them together. But then I realized that in both those things, I am handing off my consciousness quite a bit. To the movie, I am immersing myself in following the story as presented by the director. As long as I am not watching with a critical eye and it’s a good movie, I let myself be immersed such that I am experiencing what the characters do. I am not holding my own consciousness, but rather it is as if a consciousness (that of the characters) is being fed to me. I wonder if, when I’m immersed in a good movie, my brain de-puffs somewhat because I’m not having to hold my consciousness so vigorously.
It’s Pretty Life-Changing
I still remember the exact moment in early 2014 during a podcast when I learned why we sleep. I stood there, literally with my mouth open. Everything about my experience with sleep and lack of sleep suddenly made sense. All the studies about the dangers, short-term and long-term, of lack of sleep made sense.
A mystery for the ages was solved. And with this knowledge updating my mental model, I changed how I live. I made sleep a priority.
Because when you think that lack of sleep is just affecting body tiredness or how much you’ll yawn the next day, you can dismiss sleep pretty easily.
But when you realize you’re poisoning the core of who you are, you wake up.