How and Why to Optimize your Circadian Rhythm

Our body has an internal rhythm (called our “circadian rhythm”), performing different functions at different times of the day and night. In our busy modern lives we are often unaware of this rhythm and many of our lifestyle habits interfere with our body’s ability to maintain a consistent rhythm. Optimizing our daily habits to create a strong circadian rhythm can improve our sleep, alertness, metabolism and overall health.

A circadian rhythm is a change in an organism’s physiology that follows a predictable pattern over the day and night (“circa” means around, “dian” means day.) Our circadian rhythm is determined by genetics, our age, and cues from the external world. The strongest external influences on our circadian rhythm are sunlight and environmental temperature, but the timing of our sleep, meals, exercise, and social interactions also play a role. In the absence of these environmental cues (imagine living alone in a cave lit continuously by candlelight with unlimited access to food and water), humans still maintain a predictable pattern of activity, but the cycle is not exactly 24 hours long. The length of this innate cycle is the genetically determined part of our circadian rhythm and in most people it is very slightly longer than 24 hours (24.18 hours on average, see https://www.ncbi.nlm.nih.gov/pubmed/10381883 for more information.) Without input from the external world, our rhythm slowly becomes misaligned with the world. Having strong and predictable cues from the external world keeps our cycle synchronized with the turning of the earth and also seems to optimize our overall physiology, allowing us to sleep better and be more alert during the day. Understanding and optimizing environmental circadian cues can therefore help us optimize our health.

Light:

The strongest environmental circadian cue is light. Nearly all organisms evolved to be in the sun from sunrise to sunset and relative darkness once the sun went down. Consequently, nearly all organisms have a mechanism to track the timing and brightness of sunlight and modify their physiology accordingly. In humans, the backs of our eyes contain special cells, called intrinsically photosensitive retinal ganglion cells (ipRGCs). These cells detect the intensity of sunlight and send a direct signal to a “master clock” deep within our brain (the suprachiasmatic nucleus). IpRGCs are exquisitely sensitive to the blue-green portion of the visible light spectrum because the intensity of blue-green light changes the most as the sun moves in the sky, with little blue-green light at sunrise/sunset, lots of blue-green light at noon. The “master clock” in our brain uses the signal from our eyes to regulate the release of hormones into our blood. The levels of these hormones oscillate over the course of 24 hours to let the organs inside our body know the time of day.

Figure 1. Sunlight’s effect on the circadian rhythm. When our eyes detect blue-green light, they send a signal directly to a “master clock” deep within our brain which regulates the release of hormones into our blood, letting the rest of the body know the time of day. (Source: https://www.researchgate.net/figure/The-circadian-clock-in-mammalsa-A-hierarchical-organization-of-the-circadian-clock_fig3_221682652)

One of the hormones which is regulated by the ipRGC pathway is melatonin. Melatonin is a “darkness hormone” which is supposed to be released by the brain when it is dark outside. Nocturnal animals are most active when melatonin is circulating in their system and diurnal animals, such as humans, will be less active and also sleep best when melatonin is circulating in their blood. Activation of the ipRGC’s by blue-green light suppresses the release of melatonin into the blood. Once the sun goes down and our eyes no longer detect blue-green light, melatonin levels in the blood start to rise. Our ideal bedtime is when melatonin blood levels are maximally rising, which usually occurs several hours after sunset. Melatonin levels continue to rise until around the midpoint of sleep, then decline again as we approach the morning. Melatonin remains in our system until our eyes again detect blue-green light and our master clock actively suppresses its release. (Interestingly, caffeine also reduces melatonin levels, which is why people often consume it in the morning).

Figure 2. The effect of sunlight on melatonin levels. The release of melatonin into the blood is suppressed by sunlight. Once the sun goes down, melatonin levels begin to rise, peaking around the midpoint of sleep and declining again as we approach the morning. (Adapted from: https://www.sandhillsneurologists.com/melatonin-and-sleep/)

Before the existence of artificial lighting, we would have been exposed to intense blue-green light nearly continuously during the day (with some amount of variation depending on time of day, season, latitude), and virtually no blue-green light once the sun went down. In our modern lives, however, most of us are only exposed to low intensity blue-green light during the day (Americans are currently estimated to spend 90% of the day indoors,) and continue to be exposed to nearly the same intensity of blue-green light after the sun goes down from artificial lighting and electronic screens. This aberrant pattern of light exposure confuses our brain and internal clock, making the brain think that it is perhaps in a perpetual northern latitude summer, or, that the sun sets and rises much later than it actually does. Too little daytime light and too much evening light can lead to difficulty falling asleep, reduced sleep quality, difficulty waking up in the morning, daytime sleepiness, fatigue, depression and a host of other problems. Even if a person does not report difficulty falling asleep or reduced sleep quality when they use an electronic screen or bright room lighting before bed, close examination of their sleep during a formal sleep study will still reveal disruption.

Figure 3. The effects of the visible light spectrum on melatonin. The cooler parts of the visible spectrum all inhibit the release of melatonin to varying degrees. (Source http://www.newscientist.com/articleimages/mg21328503.100/1-sleep-how-much-we-need-and-what-keeps-us-awake.html)

In addition to modulating our circadian rhythm, sunlight has direct effects on our level of alertness. Bright, full spectrum or blue-green light actually causes dopamine to be released in the brain in a similar manner to prescription stimulant medications such as Ritalin and Adderall. In bright light, our reaction time improves and our mood improves.

The ultraviolet portion of sunlight has additional health benefits: it allows our skin to create vitamin D, and it lowers our blood pressure. Of course ultraviolet light also has the potential to cause DNA damage in the skin which can lead to skin cancer, so obtaining the proper balance of sunlight exposure and sun protection is important (see https://www.outsideonline.com/2380751/sunscreen-sun-exposure-skin-cancer-science for more information and guidance.)

Figure 4. The color spectrum emitted by various forms of lighting. All the lightbulbs types listed above emit significant amounts of blue-green light, though LED lights generally emit more than incandescent bulbs. (Source: https://www.sunlightinside.com/light-and-health/natural-light-is-fundamentally-different-than-artificial-light/)

You can use your new understanding the biologic effects of sunlight and blue-green light to create a robust internal clock and improve your sleep and alertness. You should try to obtain bright, full-spectrum light (such as sunlight) in the morning and throughout the day and minimize your exposure to the blue-green part of the visible light spectrum for 2–3 hours before your desired bedtime. (I will provide some more specific recommendations at the end of this article.)

Sleep Schedule Regularity:

The time that we go to sleep each night and wake up each morning has an effect on our circadian rhythm. This effect is largely independent of our pattern of light exposure. If we stay up later and sleep in on weekends, our circadian rhythm shifts, re-adjusting our melatonin timing to later. This means that when we try to go to sleep early again on Sunday night and wake up early for work or school on Monday morning, our brain thinks it is too early to go to bed and too early to awaken. (This phenomenon of delaying our circadian rhythm on weekends is called “social jetlag.”) By the end of the work/school week our circadian rhythm will have again shifted a bit earlier, but each time we stay up late and sleep in it will shift later. This chronic shifting of our circadian rhythm is a biological stressor which can contribute to obesity, diabetes, cancer, and other health problems. In addition, chronically shifting our circadian rhythm means that on several nights of the week we will be sleeping or attempting to wake up at a time which is not optimal. On these nights and days our sleep quality and alertness will be reduced.

Figure 4. Social Jet Lag. Staying up late and sleeping in on weekends shifts the timing of our circadian rhythm, making it difficult to fall asleep on Sunday night and difficult to wake up on Monday morning. (Source: https://www.researchgate.net/publication/224821917_Circadian_and_Wakefulness-Sleep_Modulation_of_Cognition_in_Humans)

An ideal sleep schedule is therefore one which is the same on both weekdays and weekends. There are numerous studies showing that people who keep the same sleep schedule during the week and weekend generally have improved daytime performance compared to people who shift their schedules. (Of course duration of sleep is also important for overall health and alertness and most people need at least 7–9 hours in bed each night to obtain sufficient sleep. If there is no possible way for you to get more sleep on weekdays, it may be helpful to sleep longer on weekends, but you should try to still keep the same or even an earlier bedtime.)

Temperature:

In humans who live without artificial lighting and without climate control, the timing of sleep onset and offset is determined primarily by changes in environmental temperature. Prior to a 2015 study which made this discovery (Yetish G, Kaplan H, Gurven M, et al. Natural sleep and its seasonal variations in three pre-industrial societies. Curr Biol. 2015;25(21):2862–2868), it had been assumed that the timing of sunrise and sunset was the primary determinant of human sleep timing. It turns out that our ancestors did not go to sleep when the sun set and awaken at sunrise; they actually went to sleep when the environmental temperature started to maximally fall and woke up right when the temperature first began to rise in the morning. Interestingly, human core body temperature varies by about 1 degree celsius (1.8 degrees fahrenheit) over the course of 24 hours, and the pattern of this temperature rhythm closely follows the pattern of changes in environmental temperature.

Figure 5. Graph of environmental temperature and core body temperature over the course of 24 hours. Our core body temperature varies by approximately 1 degree celsius over 24 hours in a pattern that is similar to that of outdoor environmental temperature. (Source: http://www.hymet.com/ClimateThermometer.aspx and https://www.physiologyweb.com/figures/physiology_graph_Ij6Z90OE6GA958p7vKhTlreCujqLSbv9_circadian_rhythm_of_core_body_temperature.html)

Because it is a relatively recent discovery that environmental temperature has such a strong influence on sleep timing, we do not yet fully understand how to optimize our home and office thermostat settings. Nevertheless, it has been known for years that sleeping in a room that is too warm reduces sleep quality, and that heating up the body with a bath in the late evening can make it easier to fall asleep. (When we get out of a warm bath or shower our body temperature falls, accentuating the natural fall in body temperature which occurs close to sleep onset.) A cool sleeping environment does not mean that you need to feel uncomfortably cold; it turns out the cooling effect is most important on the face. You can bundle up in the evening as your room temperature starts to fall and you can sleep under warm blankets to stay comfortable. Cold air on the face after waking up also seems to provide a wake-up signal to the body (environmental temperature at our natural wake-up time is supposed to be very close to the minimum environmental temperature over a 24 hour period.)

Consider trying to set your thermostat to follow a pattern similar to that of the outdoors: start to cool your house slightly in the hour or two before bed and let your bedroom temperature continue to fall over the course of the night. Avoid increasing your bedroom temperature until your desired wake time and try to keep it cooler in the hours after waking compared to later in the day.

Minor contributors to our circadian rhythm:

Meal timing, exercise timing and timing of social interactions also have effects on our circadian rhythm. In particular, our intestine and metabolic hormones seem to have their own circadian rhythm which may not be synchronized with the rest of the body’s circadian rhythm. A general guideline is to keep predictable meal times and avoid eating and intensive exercise when melatonin is circulating in your system (ie: 2–3 hours before your typical bedtime and before your typical wake-up time.) Social interactions in the morning can also help us wake up more easily.

Recommendations:

Paying attention to the environmental cues that influence your circadian rhythm and making them as strong and consistent in timing as possible can help improve sleep, alertness and overall health. Consider trying to incorporate the following elements into your daily life:

  • Start your day with a gentle walk outside. The bright light, cold air and gentle exercise will all help to strengthen your circadian rhythm.
  • Make your house as bright as you can manage in the morning. If the weather is nice, eat breakfast outside. At least 30 minutes of very bright light within the first 1–2 hours of waking is the minimum amount recommended to stimulate your circadian rhythm.
  • Try to get bright, full spectrum light throughout the day either via direct sunlight (you can still wear sunscreen, a hat, and sunglasses if these are needed to protect against ultraviolet radiation), sitting near a window, or using bright indoor full spectrum lighting.
  • For 2–3 hours before your desired bedtime, avoid blue-green wavelengths of light as much as possible by using warmer color light bulbs (such as antique-style LED bulbs, bug lights, or lights labeled as “sleep friendly”) and avoiding electronic screens. If you must use electronic screens, consider wearing blue light blocking glasses to filter out the blue-green light emitted by the screens. (Apple’s “Night Shift” feature is not sufficient to prevent the suppression of melatonin by the light emitted from its screens. Turning on Night Shift to the warmest setting is probably better than not enabling it, but you will still be reducing your evening melatonin levels by using that screen in the evening.) Instead of using a screen, consider reading with an amber-colored book light for the hour before bed.
  • Go to bed and wake up at approximately the same time every day, allowing for 7–9 hours between your bedtime and wake time. A good guide for most people is to choose a bedtime and wake up time that are 8 hours apart and try not to vary from those times by more than +/- 30 minutes (for instance, target a bedtime of 9:30–10:30 PM and a wake-up time of 5:30–6:30 AM.)
  • Begin to cool down your house and bedroom one to two hours before bedtime and continue to let the temperature fall over the course of the night.
  • Avoid warming your house significantly before you wake up in the morning. Keep the house cooler in the morning compared to later in the day.
  • Try to keep a relatively consistent meal schedule on weekdays and weekends and avoid eating for 2–3 hours before your typical bedtime and for at least 30 minutes after your typical wake time.
  • Obtain exercise daily but not prior to your typical wake time or 2–3 hours before your typical bedtime.
  • Try to engage in social interactions in the morning. If you cannot, try listening to a familiar voice on a radio or TV program, or try listening to uplifting music.
  • Note: You can also use the above strategies to shift your circadian rhythm. For instance, if you are waking up later than desired, try avoiding all blue-green light for 3–4 hours before bedtime, getting very bright light right on awakening (or even before awakening by leaving the window shades open), setting the thermostat to start warming the house 1–2 hours before your desired wake time, eating food as soon as you wake up etc.

Stay Tuned: In future posts I will provide more information about normal sleep and suggest more techniques to improve your sleep and daytime function.

About the Author: I am a board certified Sleep Medicine physician in Redwood City, California. My clinical focus is on the non-pharmacologic management of sleep disorders using techniques such as circadian rhythm optimization, bright light therapy and Cognitive Behavioral Therapy for Insomnia. I believe that understanding the structure of normal sleep and understanding how our ancestors slept in the days before artificial lighting is essential to learning how to sleep better.