temperature is a good example. You might remember from high school health class that our “natural” temperature is 98.6°F. But that’s just a baseline, as our body temperature fluctuates throughout the day. During our waking hours, it gradually rises several degrees, hitting its highest point around 4:00 to 6:00 p.m. The body’s lowest temperature comes in the early morning, hours before we wake up.1 That’s useful to know, as we feel our most energetic as our body temperature increases throughout the day, and we feel more relaxed and sleepy as our temperature falls in the evening and nighttime.2
Decreasing temperatures signal to your body that it’s time to slow down and prepare for sleep. We sleep in cycles, too, all of which are roughly ninety minutes long. When we enter our most intensive, rapid-eye-movement (REM) periods of our sleep, the cells controlling our temperature temporarily deactivate.3 That means our surroundings determine our body temperature during REM. If you’re missing some needed shut-eye, or if you wake up frequently at night, it might be that your bedroom is too warm.4 Consider lowering the thermostat a few degrees. Or, better yet, ditch those pajamas and sleep au natural. Unfortunately, modern environmental exposures, as well as stress and anxiety, can influence and disrupt these endogenous rhythms as well.
In addition to internal, self-activating cues, our bodies also have external or exogenous rhythms, biological cycles that harmonize with external stimuli. Such external stimuli are referred to as zeitgebers, from the German word meaning “time givers.” Zeitgebers help to keep the biological clock synchronized to a twenty-four-hour period and include environmental time prompts such as sunlight, food, noise, and, important for us as social animals, interaction with others.5 For instance, your internal sleep/wakefulness cycles are synchronized with the external light exposures of day/night. Get too much light at the wrong time, and you’ll have problems. We’ve all heard that we should avoid screen time before bed, and it’s not to ruin our fun. Light directly interferes with our sleep, and research suggests that such late-night exposure might be linked to cancer, obesity, diabetes, and cardiovascular problems.6
Dr. Wendy Troxel, a senior behavioral and social scientist at the RAND Corporation, contends that partnered sleeping can help the body release zeitgebers and keep us in healthy rhythms. “Partners can be very helpful to help enforce consistent sleep and wake routines,” she said. “It becomes a reminder to go to bed instead of staying up late playing video games or binging on Netflix.”7 Though Dr. Troxel concedes that many studies associate sleeping alone with a better night’s sleep, those same studies reveal that people strongly prefer social sleeping. “Prioritize sleep as a couple. Think of it as an investment in your relationship, because you really are a better partner as well as more productive and healthier and happier when you sleep better,” Dr. Troxel said.8 Whether we opt for social or solitary sleep, we must ultimately curtail our late-night light exposure and develop other healthy sleep hygiene practices to keep our internal cycles harmonious.
You’ve probably heard of circadian rhythms, a Latin word meaning “about a day,” which refers to our bodies’ core endogenous rhythm. This twenty-four-hour internal clock is the same as the time it takes for the earth to rotate on its axis—a rhythm so fundamental that it has made its way into our genetic codes.9 Your circadian rhythm coexists and moves in harmony with many (if not most) physiological and behavioral processes that occur, cyclically, every day of your life. These include your sleep/wake cycles, levels of alertness, body temperature fluctuations, blood pressure variations, reaction times, hormone secretions, digestion, and bowel movements.10
On a local level, many of these functions themselves run on shorter, recurring ultradian rhythms. Think of sleep, which happens in approximately ninety-minute cycles throughout the night. As it turns out, ultradian cycles of around ninety minutes operate during the daytime too.11 Most of us can’t maintain optimal productivity for over ninety minutes at a time, as our energy and creativity naturally ebb after such sustained focus. Instead of another cup of coffee, or disciplining yourself to keep your “nose to the grindstone,” try taking a break after every hour and a half of work.12 You might find yourself refreshed and recharged, ready for another ninety minutes of focused work.
Digestion, appetite, blinking, and sexual arousal also run on shorter rhythms. Some of these processes happen with precision and regularity. Others, like our hunger or the cycles of our moods, are more mysterious. Either way, these automatic processes tend to fade into the background as we go about our lives. We don’t pay any real attention to them until they become disrupted in some way. And then, as we’ll see, we frequently are forced to pay a lot of attention.
The Neuroscience of Rhythms
Have you heard about a “master clock” in humans and other mammals that helps to govern our rhythms and biological processes? It isn’t simply a metaphor, but a real place in the brain’s hypothalamus, located in a cluster of about twenty thousand nerve cells known as the suprachiasmatic nucleus (SCN). Situated right above the optic nerve, the SCN is sensitive to external stimuli, which help it keep accurate “time.”13
In the absence of external zeitgebers, this master clock maintains a free-running rhythm of between twenty-four and twenty-five hours—your intrinsic circadian rhythm.14 Because the shift from dark to light/light to dark happens twice per day, this synchronized rhythm is also called the diurnal rhythm. We synchronize ourselves to the daily rotation (rhythm) of the planet in step with our diurnal rhythms. But as I’ll discuss later in the book, the way we behave in the modern world often disconnects us from this planetary synchrony. The diurnal rhythm becomes disrupted if our SCN receives a light signal when it is expecting darkness, or darkness when it is expecting light. We all know what it’s like to feel groggy after a long flight—our diurnal rhythms are out of sync, and in conflict with external stimuli.15 People who habitually reverse their natural diurnal habits, by working the night shift, for example, also experience disruptions in sleep and, as we’ll see, are even more vulnerable to chronic diseases.16
You might have heard about the hormone melatonin and wonder how it fits into this rhythmic mix. When night falls, and our SCN no longer senses light, it signals the pineal gland to produce and release melatonin. As this hormone gets into the bloodstream, we become less alert, and the idea of sleep becomes much more enticing.17 We’ve all been there! Melatonin is classically thought of as a sleep hormone, but as you will read later, it is perhaps best thought of as the darkness hormone. Our bodies release it in the absence of light, and for most of us, sleep happens to occur during this time.18 You’ve likely seen it advertised in grocery market aisles and on Amazon—it’s the only hormone authorized for sale without the need for a doctor’s prescription.19 Since melatonin is linked to the dark of night, the longer the night lasts—as it does through fall and winter—the longer our exposure to melatonin. Conversely, the long days and relative short nights of spring and summer shorten the duration of melatonin output released at night. (Unless, of course, we’re popping the melatonin we’ve purchased from the pharmacy like they’re breath mints!)
Melatonin, then, is the chemical messenger that links the expanding and contracting cycles of light in our external environment to our own master clock. Melatonin plays a role, for example, in synchronizing digestive secretions and enzyme pulses, as well as periods of immune system activity. Even your skin has its own diurnal rhythm, exhibiting greater resilience to UV radiation early in the day.20 Melatonin allows the brain to orchestrate our seasonal rhythms and the biological processes that are best synchronized with these rhythms.21 But when we expose ourselves to light at night, even in small amounts, melatonin production declines and all these processes become unaligned. Yes, that means that creature comforts like a night-light, and the innocent-looking light emissions from our electronics and consumer appliances, might be imperiling our sleep—and, by extension, our health. Small things can make big differences.
A Seasonal Model of Health
