“Nature does not hurry, yet everything is accomplished.” Lao Tzu
Human vision is one of the sharpest in the animal kingdom. Our eyes help us see with high resolution but are also sensitive to light, making it hard for us to see in the dark. As such, we evolved to be awake during the day and asleep at night. That worked well for thousands of years until our big brains, which endlessly drive us to fiddle with the natural order of things, found a way to overcome nature and control the length of our day. Through the use of fire, candles, and eventually the light bulb, our ability to illuminate the night forever changed our world. Although artificial light offers numerous advantages related to our overall quality of life, it comes at a serious cost to our health. Evidence related to the brightness we have manufactured shows that, not only does it seriously impact our health, it harms the entire planet.
It is not just feelings of wakefulness and sleep that are influenced by our exposure to light. Sensitivity to light and dark regulates every physiological system in our body, from individual cells to the gut microbiome. Light exposure plays a role in regulating a number of physiological processes, including which nostrils dominate during breathing (they alternate throughout the day), kidney function, blood pressure, respiration, hormone production, and wound healing. Although we have learned ways to extend the brightness of the day, our bodies are designed to stay in sync with the rhythms of nature. Regardless, we keep inventing new ways to illuminate the night, a time when nature tells us it should be dark.
Most of our cells have their own specialized chemical timekeeper that keeps them in sync with their own rhythmic ebb and flow of energy. To coordinate all the functioning of our various physiological systems, we have a “master clock” called the suprachiasmatic nuclei, or the SCN, which is located deep in the brain, in the hypothalamus. Specialized cells in the retina convey information about light levels to the SCN, which adjusts our overall body rhythm accordingly. Information about ambient light is also transmitted to the pineal gland. When light levels decrease, the pineal gland starts to release melatonin, a hormone that promotes sleep. As darkness increases, melatonin release rises, triggering the physiological changes that prepare us for an important part of our day, the work of sleep.
Over the years, we have sharpened our understanding about the architecture of the sleep cycle. We have known for a long time that sleep unfolds in stages, but imaging is showing that each stage corresponds with a specific function that impacts our brain health. A description of each of these stages is beyond the scope of this discussion but be aware that each stage serves a series of specific tasks, from memory consolidation, our ability to learn new things, the retention of information, and the removal of metabolic toxins from the brain and spinal cord. Whereas we once believed we could “catch up” on lost sleep, perhaps sleeping in on a weekend after several days of all-nighters, we now know that each stage is equally important and takes time to complete. There is no catch up, nor are there shortcuts to the work required of each stage. The bottom line is that poor sleep leads to decreased functioning of our mind and body.
The CDC estimates that approximately one third of Americans are chronically sleep deprived, which is measured by getting less than 7 hours of sleep in 24 hours. The recommended amount of sleep for most adults is around 8 hours per night. This number is not arbitrary. The work of sleep is to move through each stage completely, as our brains need sufficient time to complete the required task. Our sleep needs change throughout our lifetime and, although there are slight individual differences, most studies confirm that a range of 7 – 9 hours per night for adults is considered optimal. Sleep seems to follow the “Goldilocks’ Principle”—we need just enough, not too much and not too little; staying within this range is optimal. All of us occasionally experience days or periods of time when sleep is elusive, but chronically falling outside of the 7 – 9 guideline has consequences.
I’ve heard people say that they will “sleep when they die”. That day may come sooner than expected. Many of us consider sleep a luxury. It is not, however, something we do only when we have time, but rather an essential function that is a critical piece in the cycle of our day. People who sleep less than 5 hours or more than 9 hours per day consistently show increased markers associated with chronic disorders such as diabetes, cardiovascular disease, and obesity. We have known for a long time that dysregulated sleep patterns are part of depression and anxiety disorders, but growing evidence suggests that a lack of sleep may be fertile ground for the development of these conditions. It seems our immunity takes a direct hit when sleep is insufficient, which was illustrated quite clearly during the pandemic. People who were chronically sleep deprived were much more likely to be infected with COVID and suffered poorer outcomes once sick. And one of the most profound associations between sleep and brain health relates to cognitive decline as we age. Mid-life insomnia is associated with an increased risk of Alzheimer’s disease, and a history of poor sleep increases one’s risk of developing late life cognitive impairment.
An occasional interruption in sleep is not cause for concern, but, overall, our body’s need for sleep is a rigid taskmaster. Just as important as the amount of sleep is the quality of our slumber. Simply meeting the total number of recommended hours is not enough if one is waking up frequently throughout the night. Disruptions in the sleep cycle interfere with the work of a given stage, requiring the brain to cycle back to earlier stages. Frequent waking results in less time spent in stages during which important processes take place. These include the dreaming stages (REM), that are related to emotional regulation, and slow wave sleep during which our memories are consolidated and the ability to learn is sharpened.
Some of the most compelling evidence of the body’s need for sleep comes from epidemiological studies that show a connection between insufficient sleep and the development of obesity and overweight. Scientists have known of the connection between weight and poor sleep for many years, but the assumption was that conditions related to obesity, such as higher levels of fat tissue in the neck region, lead to sleep apnea and lower sleep quality. But newer evidence suggests sleep can change our relationship to food—our choices, how much we eat, and our preferences. Clinical studies of adults showed increased reports of hunger and a higher caloric intake when subjects were sleep deprived. Poor sleep is also associated with a preference for late night food intake of sugary and processed foods (think pizza, chips, ice cream) and more frequent snacking. And the less sleep we get, the more we seem to prefer higher carbohydrate and fatty foods. It seems insufficient sleep activates the reward centers in our brain, making junk foods more attractive than carrots or an apple. And sleeping only four to five hours a night makes this drive even stronger, creating more feelings of hunger while increasing our cravings for sweet foods. This comes as no surprise to all those sleep deprived new parents, college students, shift workers, or those with medical conditions that make restorative sleep elusive.
The primary driver in these poor eating patterns is related to the connection between appetite hormones and sleep deprivation. The pattern emerging from all the sleep research is that deprivation puts the body under chronic levels of stress. One of the ways our endocrine system responds to this type of stress is to shift our energy needs, dropping our levels of leptin, the hormone that signals the body to stop eating, while elevating ghrelin, the hormone that sends hunger signals to the brain. The result is that we feel hungry and want to eat more. Additionally, there seems to be gender differences in these hormone shifts. Sleep deprivation in men stimulates an increase in ghrelin, whereas women experience lower levels of leptin. Men have a greater appetite and desire to eat while feelings of satiety are blunted in women.
Although studies have shown a correlation between poor sleep and diet, they do not determine causality. And much of the research failed to identify whether sleep was the chicken or the egg. Does our ability to make healthy food choices depend solely on our sleep levels? And can poor food choices interfere with our ability to sleep? Newer research suggests that the relationship between diet and sleep is a two-way street: the foods we eat affect how well we sleep, while the quality of our sleep influences our dietary choices. Although we are far from knowing which diets are best for sleep, studies point to the overall quality of how and what we eat throughout the day as being more important than one particular food item or meal. Further research is needed but some large studies found an association between better sleep and the consumption of a Mediterranean diet that emphasizes vegetables, fruits, nuts, seeds, legumes, whole grains, seafood, poultry, yogurt, and olive oil.
Before we rush to find a diet that guarantees a perfect night’s sleep, experts express caution in drawing too many conclusions from what we know so far. What we do know for certain is that diet and sleep are entwined, and that improvements in one will improve the other. What we don’t yet know is whether it is a specific nutrient or combination of foods that promote restorative sleep. And there is evidence that not all people experience significant increases in weight when sleep deprived; some do tend to eat more junk food yet do not gain any weight. Still, the impacts of chronic sleep deprivation have a long reach. Some epidemiologists are making a connection between a growing obesity epidemic, rising levels of inflammation, poor sleep patterns, and a lack of darkness in our modern world. Perhaps we overestimate the importance of daylight and underestimate the long hours of darkness at our own peril.
It seems science keeps challenging our culture’s long held assumptions about weight, telling us that fat reflects much more than “calories in, calories out”, a lack of focus, or a deficit of willpower. Our relationship with food is complicated. It is infused with emotions and sensations, coupled with an appetite that is influenced by many factors outside of our awareness, one of which is the quality of our sleep. Add to this mess an independently operating microbiome that calls the human gut its home and you have a jumbled mess of factors, some of which we can change and others that are beyond our control.
If you aren’t finding the right answer, then it is time to ask a different question. Rather than search for the perfect diet, what if we looked to how we might live more in sync with the natural rhythms of the world and of our bodies? Can we learn to appreciate the restorative nature of the night as much as we do the bright busy-ness of the day? What happens to our health when we turn inward and listen, rather than endlessly analyzing, tweaking, and measuring ourselves, watching our steps, calories, sitting, exercise, and so on. Perhaps we might find answers by turning to the work of the night, becoming passive bystanders that are restored with sleep. For that, we need only to let go of expectations and allow the body to do its work.
Anyone who suffers from chronically disrupted sleep—of which I am one--knows that such suggestions are soft and squishy as well as very hard to implement. But maybe the best way to improve our health starts by paying attention to the darker side of life, that mysterious place where we go to be restored. What would happen if you shifted your attention away from all the things you are doing, focusing instead on how you are not doing—that is, just being at rest—and allowing yourself to let go? Who knows where that question will take us, but I am certainly up for putting to the test!
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