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Circadian Rhythm: A Key to Health
by Laura Schwecherl
The Complete Guide to Circadian Rhythm
The circadian rhythm is a 24-hour biological cycle that governs when we fall asleep and wake up. A vital part of health and wellness, the circadian rhythm also influences other essential bodily functions such as hormone release, eating habits, digestion, and body temperature. (1)
A synchronized circadian rhythm can promote better physical and mental health while helping prevent conditions such as obesity, diseases of aging, and sleep disorders. (2)
Note: The content on Sleepopolis is meant to be informative in nature, but it shouldn't be taken as medical advice, and it shouldn't take the place of medical advice and supervision from a trained professional. If you feel you may be suffering from any sleep disorder or medical condition, please see your healthcare provider immediately.
How A Healthy Circadian Rhythm Functions
The master biological clock is located in the hypothalamus in the brain, and controls the many "local" clocks spread throughout the body in organs and muscle tissue. This vast circadian network plays a crucial role in physiological and psychological functions, as well as short and long-term health.
In addition to regulating sleep, internal clocks synchronize biological processes and allow the body to function properly. A healthy circadian rhythm can improve alertness, memory, and cardiovascular efficiency (3) (4) (5), and over time, reduce the risk of chronic diseases and cognitive impairment. (6)
An optimized circadian rhythm helps support:
Regular sleep patterns (7)
Healthy melatonin, insulin, and cortisol release
Stable core body temperatures
A strong immune system
Q: How does cortisol impact sleep? A: Cortisol, also known as the stress hormone, may delay or prevent sleep if levels are too high, and make waking up difficult if levels are too low. A healthy circadian rhythm helps keep cortisol levels balanced.
Out of Sync: A Disrupted Circadian Rhythm
The circadian rhythm depends on consistent signals from the brain to maintain a healthy sleep-wake cycle. Lifestyle choices such as staying awake all night, eating a large meal before bed, or flying across time zones may disrupt the circadian rhythm, leading to numerous short-term health effects. (8) (9). These effects include:
Elevated blood pressure
Poor appetite control
Fluctuating blood sugar levels (10)
Short-term disruption to the circadian rhythm may be mitigated by a return to consistent bedtimes and normal light-dark exposure. If disruptive behaviors persist, however, the circadian rhythm may become chronically disorganized or even damaged. This type of dysfunction may be caused by:
Regular travel to different time zones
Working non-traditional hours
Use of certain drugs or medications, such as stimulants
Insomnia disorder
Sleep disorders, such as sleep apnea and restless legs syndrome
In addition to effects like daytime sleepiness, difficulty falling and staying asleep, and increased stress, chronic circadian rhythm disruption may cause long-term health conditions. Growing scientific evidence ties a damaged circadian rhythm to serious diseases including diabetes, heart disease, and cognitive decline. Depression and other mood disorders may also result, particularly if the dysfunction begins earlier in life.
A syndrome that occurs when the body's biological clock and sleep schedules don't match up, usually due to sleeping and waking up later on the weekends.
Persistent misalignment between the internal body clock and external surroundings may result in a circadian rhythm disorder. This type of disorder can cause insomnia and excessive sleepiness, and increase the risk of certain physical and psychological conditions.
The most common circadian rhythm disorders include:
Delayed Sleep Phase Disorder. This disorder is more prevalent in adolescents and young adults. People in these age groups tend to feel tired later at night and go to sleep at least two hours past a typical bedtime, around 2:00 a.m. Though they may still require 7-9 hours of sleep, work, early school start times, or other demands may force some young people to forego sleep
Advanced Sleep Phase Disorder. Advanced Sleep Phase Disorder is most common in older people who fall asleep and wake too early. Sufferers generally fall asleep between 6 and 9 pm, and wake between 2 and 5 am. People with this disorder may complain of early morning insomnia, as well as sleepiness in the late afternoon or early evening
Shift Work Disorder. Shift work disorder may affect people who work at night or frequently rotate work shifts. Because of the misalignment between sleep/wake and dark/light cycles, shift work disorder sufferers may complain of insomnia, excessive sleepiness, microsleep episodes, or depression
Untreated circadian rhythm disorders and chronic sleep loss may lead to the development of certain diseases. Some of the diseases linked to circadian rhythm disorders include:
Certain cancers
Work schedules, meal times, and light exposure all influence the functioning of the body's clocks, which in turn impact important physical and cognitive processes. According to scientists, decades of social and lifestyle changes have had widespread effects on circadian rhythm, and may be related to significant increases in cardiovascular and metabolic diseases.
Circadian Disturbance Example: Eating At Night
Using cues from the light-dark cycle, the internal master clock governs the local clocks spread throughout the body in organs and muscle tissues and signals the body to rest at night. (11) Eating around bedtime can override this signal, forcing the pancreas to "wake up" to release insulin. (12) This may create an imbalance between the pancreas and master clock, leading to delayed or disrupted sleep.
According to circadian scientists, the body's metabolism, hormones, enzymes, and bacteria are optimized for food consumption in the morning and afternoon. Metabolic health may be improved by confining meals to an eight-to-ten-hour time period, and consuming more food earlier in the day.
One study compared the effect of eating a large breakfast versus a large dinner, and discovered the large breakfast-eating group lost more weight than the group that ate a large dinner. Additionally, the dinner group experienced a release of insulin and an increase in blood pressure, which normally decrease at night. (13)
Q: When is a reasonable time to eat dinner? A: Many experts say to eat a lighter meal at least two hours before bed.
Circadian Disturbance Example: Working the Night Shift
The body's master clock evolved to follow natural light cues, which signal the brain to wake during daylight and sleep at night. (14) Night shift workers and others who work non-traditional hours may miss these signals, resulting in sleep difficulties and a desynchronized circadian rhythm.
At night, the body's core temperature drops, blood pressure decreases, alertness wanes, and the production of the hormone melatonin increases (15), promoting deep rest. Remaining awake once these processes begin may work against the body's sleep signals and throw off key biological functions.
Shift-workers often experience extreme fatigue and are at risk of developing a circadian rhythm disorder. As many as 40% of shift-workers in the U.S. suffer from Shift-Work Sleep Disorder (SWSD), a result of habitual conflict between a sufferer's circadian rhythm and work schedule. Due to the lack of alignment between the sleep/wake and dark/light cycles, SWSD sufferers are at increased risk for insomnia, excessive daytime sleepiness, and cardiovascular and gastrointestinal problems. (16) (17)
Work that takes place outside the traditional 9 am to 5 pm time frame, including early morning and night shift work as well as work with a rotating schedule.
How To Improve A Circadian Rhythm
Circadian rhythms vary from person to person, and are influenced by age, genetics, and environmental factors. Every organism has a specific chronotype, also referred to as a biological clock. Individual chronotypes are determined by genetic predisposition, as well as the circadian gene known as period circadian protein homolog 3, or PER3. (18)
Chronotype controls the circadian rhythm as well as sleep, eating, and exercise patterns. Each chronotype is considered a "circadian rhythm personality," and helps distinguish early risers from night owls. (19)
There are four main chronotypes, each named after an animal:
Bears. Bears tend to sleep and wake according to the sun. They typically feel most energetic during the day, and fall asleep easily at night
Wolves (Night Owls). Wolves generally sleep and wake later than others. They are often most productive in the middle of the day and in the evening
Lions (Early Birds). Lions wake up early and are most productive in the morning. Due to early wake times and daytime energy expenditure, they are usually tired by evening
Dolphins. Dolphins are light sleepers who have difficulty following a regular sleep routine. They are most productive in the mid-morning to early afternoon
Once an individual's chronotype is identified, a healthy and consistent routine can be established to cater to the body's natural rhythm. This may mean maintaining consistent bed and wake times, and eating meals and exercising around the same time each day, including weekends. The circadian rhythm can become disrupted when sleep and meal times differ on the weekends, creating social jet lag. (20)
Additional healthy habits can help to support a healthy rhythm. These include:
Limiting evening technology use. Exposure to artificial light from computers and other devices may be harmful to the circadian rhythm, making sleep more difficult. (21) Sleep specialists usually recommend turning off devices at least two hours before bed
Sleeping in total darkness. A dark room signals the body that it is time to sleep. (22) Minimize natural and artificial light by using blackout curtains and unplugging electronics
Going for a morning walk. Energy levels and alertness are closely tied to light. Exposure to the sun can help jump-start the circadian rhythm by sending wake signals through the retina to the brain (23)
Optimizing exercise times. Scheduled exercise, either in the morning, afternoon, or late evening, can influence the circadian rhythm. A recent study found that mice who exercised in the morning or afternoon maintained a healthy circadian rhythm, while those who remained sedentary or exercised in the late evening did not. (24) A morning or afternoon workout routine may help promote smooth functioning of the body clock
Q: What is the most popular chronotype? A: Sleep researchers say "Bears" represent 50% of the population, while "Dolphins" represent 10%.
A synchronized circadian rhythm may help diminish the risk of numerous health conditions, particularly over time. Non-traditional work hours, travel, and later sleep and wake times can disrupt the body's rhythms and increase the risk of high blood pressure, obesity, and other potentially serious diseases.
Though the typical modern lifestyle may not always harmonize with the body's biological clock, a disrupted circadian rhythm can be restored. Maintaining a consistent sleep schedule, rethinking meal times, and timing exercise properly can encourage a healthy rhythm, and in turn, a healthier life.
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Laura Schwecherl
Laura is a journalist with nearly a decade of experience reporting and covering topics in the health, fitness, and wellness space. She is also a marketing consultant, where she works with impact-oriented startups to build marketing and editorial strategies. Outside of work, you can find her reading Murakami novels, writing amateur poetry, or trail running in her hometown, Boulder, Colorado.