The Science of Sleep, Part II: The Circadian Rhythm
In this article, we will walk through the stages of the circadian rhythm and learn how to reset our clocks manually.
By Kaitlin Meyer — December 14, 2022
For ages, human activity was governed by the sun, but by 1912, when New York was dubbed the "city that never sleeps," the sun's iron rule had been usurped by incandescent light bulbs, oil lanterns, and gas lamps.
The governance of mankind's illuminating inventions extends into today, particularly into the life of college students: from early morning classes to late night study sessions to Friday night parties, it's safe to say that most students don't rise and sleep by the sun. In this age of artificial lighting, we must be aware that our biological clocks, formed during our long evolutionary history, govern our physiological processes.
When it comes to waking up and falling asleep, the human body is on its own time. Even if you wake up at the same time every morning, you may not feel sufficiently rested and may have trouble falling asleep at an appropriate time. This could be caused by some "incorrect settings" in your internal clock: the circadian rhythm. In this article, we will walk through the stages of the circadian rhythm and learn how to reset our clocks manually.
How it Works
Let's first look at how our biological clock works and what triggers it to change. The circadian rhythm is controlled by "zeitgebers," or "time givers," which signal to the body that it's time to wake up, eat, exercise, sleep, or perform other functions. Light is the most potent and effective zeitgeber, but others include temperature, food, exercise, and social activities.
When light photons hit the retina, they signal the “master clock” in the hypothalamus. The hypothalamus, which regulates most of the hormones in your body, then inhibits the production of melatonin and begins the production of epinephrine and cortisol. This release of hormones starts a chain reaction, ensuring your metabolism, digestion, and other physiological processes are on schedule. When the retina stops receiving light at night, it inhibits the production of cortisol and epinephrine. It activates melatonin production, thus beginning a different chain reaction and increasing your drive to sleep.
The important thing to note is that your cortisol levels peak sometime after you wake up. The earlier this happens, the earlier you will feel sleepy. Similarly, melatonin levels peak about three hours before you wake up. The balance of these diurnal hormonal peaks should be kept; otherwise, your master clock becomes out of sync. When you experience several poor nights of sleep or have trouble falling asleep and sticking to a consistent schedule, your master clock is likely out of whack.
Confounding the Clock
One thing that may disrupt the balance is a chronic stress response, which releases too much cortisol and epinephrine for too long. You will notice your digestion going out of whack, your metabolism slowing, and your appetite deviating from standard patterns. An overload of mental stressors, inflammation, lack of sleep, and other factors can cause a chronic stress response. When you experience these symptoms, it's time to reset your clock, bringing your body back into alignment with your master clock and, thus, with your environment.
Resetting the Clock
The first and best way to reset your clock is by getting a full night of sleep. This will allow your body to catch up on digestion and repair itself sufficiently to take another day. You can do several other things to help, which will also contribute to getting good sleep! The frequency and timing of meals throughout the day can also affect your circadian rhythm. Eating at regular times helps send the correct timing signals to your body. Try eating during the same 8-10 hours of the day and consuming your last meal 3-4 hours before bedtime. This will allow your metabolic processes to kick in and align other physiological processes with the master clock.
Next, you can leverage the most powerful zeitgeber: light. Not all light was created equal; your retinas must be exposed to natural, unfiltered light first thing in the morning. This will trigger the release of cortisol and epinephrine early in the day, making you more likely to sleep earlier. Exposing your retinas to natural light means no windows, car windshields, or sunglasses.
If it's cloudy outside, it means spending up to thirty minutes outside, so you have enough exposure to morning photons. While you should absorb as much bright, natural light as possible in the morning, the opposite is true at night: avoid vibrant overhead lights, opting for red light and sources of light that are close to the ground. To truly leverage the power of natural light, watching the sunset will help trigger the release of melatonin due to the type of light emitted and the slow dimming of the light.
Exercising early in the day can help kick your metabolism into gear and will also encourage the production of cortisol, shifting your peak cortisol production to earlier in the day. Regular exercise will support a stable circadian rhythm and, thus, a stable sleep cycle. Try going outside or a walk or a run every morning!
Check the Temperature!
Lastly, regulating the temperature of your environment can also help shift the peak production of both cortisol and melatonin. Your body temperature tends to be warmer when you are at peak cortisol levels (post-wake-up) and cooler when you are at peak melatonin levels (pre-wake-up). Cool your sleep environment by opening a window or adjusting the air conditioning. While not intuitive, taking a warm shower before bed will signal your body to cool its internal temperature. Avoid excessively hot or long showers, as these will increase your body temperature at a certain point. In the morning, exercise and cold showers will help your body raise its internal temperature, encouraging the release of cortisol at an appro priate time.
We have created a partially artificial environment for ourselves, separating us from the natural rhythms of nature: we are free from dependence on light from the sun, the climate of certain regions, the timing of our workouts, and the dangers of living in an exposed environment. While this has many benefits and has allowed great advances in our civilization, we are also atavistic biological beings who depend on the natural cycles of the sun and our revolving planet. We need to stay in tune with our origins to satisfy the new ageists and sleep well at night. Look out for our next article in the sleep science series, which will explore the stages of sleep and their implications for your sleep habits.
Kaitlin Meyer
Kaitlin Meyer is a Master's student at Ohio State University (OSU), and is writing a thesis on snow microstructure inspired by her love for skiing. She earned a B.A. in Liberal Arts from Wyoming Catholic College (WCC).
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