Sleep I: An Evolutionary Imperative

I think sleep is a crucial part of maintaining health. It is an insurance policy that is too good not to participate in. This will be the first in a series of articles discussing sleep and its importance to our overall well-being. Some of this can be considered anthropomorphizing and certainly hypothesizing, but we learn through stories. So if you would indulge for a story about sleep….

Travel back to our days as hunter gatherers. The rhythms of our day completely controlled by the light and dark cycles orchestrated by our rotation about the sun. As the sun slides down the horizon, it becomes much harder to find food. And in this ancient world of incredible competition for calories, our energy would almost always be best used in search of food. Therefore, when our ability to find food is limited, it would be beneficial to conserve our energy until we are in a situation that can leverage our unique tools developed for calorie acquisition, i.e. day time vision. From this very basic pattern of light and dark, along with a perspective of calorie conservation, we might develop two different modes of being, one of activity, and one of rest and repair.

That being said, sleep’s ability to withstand natural selection is nothing short of a miracle. Sleep is seemingly juxtaposed to many of the behaviors we know to facilitate the passing of our genes into future generations. When we sleep, we are not looking for food, we are not eating food, we are not having sex, we are not looking for a mate, and we are incredibly vulnerable. These are not trivial facts, they are pillars of what we know to be necessary for procreation. So how does something that fails to directly help us in these pursuits, while also making us the most vulnerable of prey, become so prominent in essentially every animal species on this planet? Ockham’s Razor would simply tell us that the benefit must outweigh the harm. Over the long experimental testing grounds of time, mother nature has weighed and measured sleep, and it has proven to be of essential utility. Sleep’s persistence proves its profits exceed its costs. By understanding the magnitude of what we give up through sleep (eating, sexing, security, etc.), we may begin to understand the value we receive through sleep. It simply has to be greater than or equal in value or sleep would not have proliferated.

We don’t know what all the benefits of sleep, and I’m not convinced we ever will. The system-wide effects of something like sleep are hard to tease apart in the discretizing manner demanded by modern science. However, it is being researched more and more and we will be able to increasingly understand the pieces of its puzzle. Our body is able to synchronize different processes through oscillating hormone levels. Throughout the day hormone concentrations rise and fall, creating a rhythmic balance for our cellular operations. There are numerous hormones, and they all have different effects. For example, melatonin ideally starts to increase in the evening, peaks in the middle of the night, and remains low throughout the day. The cyclic variation of hormones act as a internal clock, sending information throughout the body and allowing for different parts of the body to work towards common goals.

Two specific hormones I would like to discuss here are leptin and ghrelin. When discussing biochemistry, we will have to settle with some simplification. Keep in mind when people say something like “melatonin is the sleep hormone,” there is probably a good amount of truth to it, but there is also a vast complexity going on in the background. So while melatonin is certainly involved in sleep/wake cycles, its role is much more complex.

Leptin is a hormone primarily made by adipocytes (fat cells) and enterocytes (small intestine) that signal satiety. It is a huge part of that “full” feeling we get after eating a meal. Ghrelin is a hormone produced by your gastrointestinal system, closely correlated with our sensation of hunger. These two hormones have opposing effects, and are largely involved in appetite regulation. For example, ghrelin is often at its highest concentration before a meal and at its lowest levels after eating. The opposite is true for leptin, as its concentration is highest after eating.

Let’s look at how these hormones are affected by sleep. One of the most common ways to study something is to remove it, and then observe or measure the effect of its absence. Many studies have shown that when we are sleep deprived, the circulating levels of these hormones are changed. One study took a small group of participants and took them through two different scenarios. In the first part of the experiment the participants underwent two days of sleep restriction, then had blood levels of ghrelin and leptin measured, along with a subjective assessment of hunger. These same participants where then later allowed two days of extended sleep, and the same measurements where recorded. The study showed that after sleep deprivation, levels of ghrelin increased, levels of leptin decreased, and subjective hunger was increased [1]. Another study looked at a much larger cohort of patients over a longer period of time. Here they showed that short sleep duration was associated with higher levels of ghrelin and lower levels of leptin, independent of BMI, age, sex, and other confounding factors [2]. In this review article, researchers looked at the body of evidence regarding sleep loss and its effect on neuroendocrine and metabolic function, concluding short sleep is associated with an up-regulation of appetite, lower leptin levels, and higher ghrelin levels [3]. There are numerous other studies out there, and there seems to be a strong general consensus that shortened sleep is associated with lower leptin, higher ghrelin, and increased feelings of hunger. Obviously this is a bad combination for anyone who is concerned about their weight, and an extremely difficult situation to overcome if one is trying to lose weight.

Allow me to step back from the science, and return to our hunter gatherer ancestors to try and tell a story. I do not think it is a huge leap to assume that sleep was something we engaged in every night, and something we rarely sacrificed. If not for any other reason than our gift of vision was severely limited without the light of day. However, I can imagine at least one scenario when we would sacrifice sleep. Those nights when we were on the verge of starvation, when we had gone many days without food. At that point we had no other option but to continue moving in search of food, or at least significantly shorten the time we spent asleep. So if we were on the search for food, bargaining sleep for more exploration time, how might our bodies help us? We would be at a huge advantage if our appetite was tuned for high caloric intake. That way if we managed to finally come across food, we could fully take advantage of the available calories. We would not want to be forced to stop eating because we felt “full.” In this situation it would be a great development if in response to short sleep, our body increased its signal for hunger, and decreased its signal of satiety. Increased ghrelin and decreased leptin, in order to increase our appetite and ability to intake large amounts of calories. Shortened sleep would increase the instinctual drive to find calorically dense food.

Of course this is not science, the evolutionary story may or may not be true. However, viewing things through and evolutionary lens allows us to expand our thinking to why things might work as they do, and I certainly remember things better in story than factual bullet points. So take the evolutionary part with a grain of salt, but the elevation of ghrelin, reduction in leptin and overall increase in hunger in response to short sleep is well understood. If you or anyone you know is struggling with their weight, sleep is an essential first pillar to attack. Leptin and ghrelin are only part of this story. Short sleep also impairs glucose tolerance and causes other hormonal imbalances. Diet and exercise are what people often jump to when discussing weight control, but I would argue sleep should be the first stepping stone. Without prioritizing sleep you will be fighting an uphill battle. Stay tuned for further exploration of sleep’s wide ranging effects on our health.

Best explorations

References:

[1] Spiegel K, Tasali E, Penev P, Van Cauter E. Brief communication: Sleep curtailment in healthy young men is associated with decreased leptin levels, elevated ghrelin levels, and increased hunger and appetite. Ann Intern Med. 2004;141(11):846‐850. doi:10.7326/0003-4819-141-11-200412070-00008

[2] Taheri S, Lin L, Austin D, Young T, Mignot E. Short sleep duration is associated with reduced leptin, elevated ghrelin, and increased body mass index. PLoS Med. 2004;1(3):e62. doi:10.1371/journal.pmed.0010062

[3] Van Cauter E, Holmback U, Knutson K, et al. Impact of sleep and sleep loss on neuroendocrine and metabolic function. Horm Res. 2007;67 Suppl 1:2‐9. doi:10.1159/000097543

-Ryan; 6/1/2020

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