Circadian Rhythm

author

Medically Reviewed by

Dr. Joshua Tal

Written by

Danielle Pacheco

The human body follows an internal timekeeping system known as a circadian clock. This internal clock regulates the body’s natural “circadian rhythm,” your daily cycles of sleep and wakefulness, hunger and digestion, hormonal activity, and other bodily processes. The word circadian comes from the Latin phrase “circa diem”, meaning “about a day”, referring to how most circadian rhythms automatically reset every 24 hours. Circadian rhythms are guided by natural signs that you should be awake like light exposure, interaction with people, and planned meal times. However, once set, circadian rhythms can be quite difficult to change, preserving the rhythm without any exposure to the typical signals.

What Is Circadian Rhythm?

The circadian clock consists of a cluster of roughly 20,000 neurons known as the suprachiasmatic nucleus (SCN). The cluster is located in the hypothalamus at the base of the brain. When the eyes perceive light during the day, they activate signals that travel down a nerve tract to the SCN, which lets the brain know it is time to be awake. The SCN then releases a series of hormones, including cortisol, making sure you are awake and perky for your 9:00 am meeting.

The body uses light and other signals, called ‘zeitgebers’ (German for “time giver” or “synchronizer”) to determine whether it is day or night and to synchronize circadian rhythms accordingly. Light is considered the most important zeitgeber for the circadian rhythm. Even when our eyes are closed, the eyes still perceive light and activate signals to the SCN. Other zeitgebers include physical activity, food intake, body temperature, and social interaction.

Circadian rhythms regulate the production of different hormones throughout the 24-hour cycle. When the sun rises in the morning, the body produces cortisol, a hormone that makes us feel refreshed and alert. After waking, a healthy person will become increasingly tired throughout the day until the sun goes down, when feelings of tiredness peak. As the sun begins to set, the pineal gland will release melatonin, a hormone that reduces wakefulness and alertness.

Circadian rhythms also regulate hunger and digestion, body temperature, mood, fluid balance, and other important physiological processes. For most healthy adults, the circadian clock will reset every 24 hours. However, there are variations on when people feel tired and when they feel alert throughout the day. Two examples are “early risers,” who go to bed and wake up early, and “night owls” who go to bed relatively late and then sleep in.

Your sleep rhythm will also evolve and change with age. For example, older people tend to go to sleep and wake up earlier in the day than younger people, while babies will sleep in multiple phases throughout the day and night.

How to Change Your Sleep Schedule

People may want to change their circadian rhythms and sleep-wake cycles for different reasons. Some need adjustments after starting a job that requires them to work late at night or early in the morning. Others find that “early riser” or “night owl” schedules do not provide enough sleep each day, and would like to adopt a healthier sleep routine. If you’re about to travel across multiple time zones, acclimating to the local time can help minimize the effects of jet lag.

Genes or late nights working the graveyard shift may incline your circadian rhythm one way or another. Some methods of changing your sleep schedule are less effective than others. Consuming alcohol is one example of a less effective strategy. Alcohol is a central nervous system depressant that induces feelings of sleepiness after it is consumed, so many people drink in order to feel more tired and relaxed before bed. However, alcohol also lowers sleep quality and duration, making sleep un-restorative and choppy.

Sleep medications are also questionable as long-term strategies. When prescribed properly, some medications can help you acclimate to a new sleep schedule or get you through a significantly stressful period that is affecting your sleep. However, sleeping pills are a temporary fix and they won’t alter your circadian clock. Additionally, some sleep medications can leave you feeling drowsy when you wake up.

In order to effectively alter your circadian rhythm and sleep-wake cycle, we recommend the following techniques:

  • Wake up every day at the same time: Keeping a regular sleep schedule will help reset your circadian rhythm. By going to sleep and waking up at the same time every day, your body will learn to adjust to the new rhythm. Even if you are unable to fall asleep at your desired time, make sure to set an alarm and wake up at the set time anyway. This will keep you on track.
  • Bright light therapy: Exposure to bright artificial lights can reorient circadian rhythms quite effectively. Timed exposure works particularly well for shift workers, or those whose job schedules include late night and/or early morning hours. Different light therapy devices are available, including lightboxes, desk lamps, and sunrise simulators. Before purchasing one of these devices, you should speak with a credentialed sleep medicine physician about the light exposure level and times of the day for exposure that are best suited to your particular circadian rhythm’s timing.
  • Melatonin supplements: In addition to the natural hormone produced in the pineal gland, melatonin is also available in supplement form. Melatonin supplements were not developed to treat insomnia, rather to help reschedule circadian rhythms when timed correctly. Taking over the counter and prescription melatonin supplements can help you wake up and go to bed at different times of the day. A standard dose of melatonin is 0.5 milligrams, and supplements should be taken a few hours before bedtime under the instruction and care of a sleep specialist. Always consult with your doctor before taking any supplements.
  • Different meal times: Circadian rhythms regulate when we feel hungry and how we digest food. Some studies have found that advancing or delaying meals can alter how your circadian rhythm regulates these processes, causing you to feel alert and tired at different times than those you’ve become accustomed to.
  • Exercise: Exercise and sleep share a somewhat symbiotic relationship. Proper exercise can improve sleep quality and duration, while a healthy sleep-wake cycle ensures more strength and endurance when you work out. However, exercise is also stimulating if you work out too close to bedtime. If you find you don’t get enough sleep at night and want to reorient your circadian rhythm, try incorporating regular exercise into your routine. But as with all things related to the circadian rhythm, timing is important so do not exercise within 1-2 hours of your bedtime.
  • Caffeine: As a short-term energy booster, caffeine can be a very effective – albeit, temporary – fix for shift workers, jet-lagged travelers, and other people who experience grogginess. Caffeine also has a half-life of 5 hours in a healthy adult, meaning it takes an average of 5 hours for the body to eliminate half the amount of consumed caffeine. For best results, consume a moderate amount of caffeine for the first few hours when you’re awake, but stop consuming it at least 5-7 hours before bed.

If you’d like to change your sleep schedule, consult with your doctor or another credentialed physician about the safest and healthiest measures that are right for your particular circadian goals.

Circadian Rhythm Sleep Disorders

The master circadian clock in healthy adults will operate on a daily cycle that resets about every 24 hours. Circadian rhythm sleep disorders are caused by delays, advances, and complete dysregulation of a person’s circadian cycle. These disorders can take different forms – though, for most, disturbed sleep and excessive daytime sleepiness are common symptoms.

Some can be attributed to a timing issue in a person’s internal timekeeping system. For instance, delayed or advanced sleep-wake phase disorder occurs when someone’s sleep-wake cycle falls at least two hours later or earlier than traditional circadian schedules. Another example is irregular sleep-wake rhythm disorder, which is characterized by fragmented sleep-wake patterns that produce disturbed sleep when the person tries to sleep and grogginess when they are awake. This disorder is often seen in people with Alzheimer’s disease, Parkinson’s disease, and other neurodegenerative conditions.

Other circadian rhythm disorders are due to misalignments between someone’s circadian clock and their external environment. Shift work disorder, a common condition among people who work late at night or early in the morning, can cause excessive daytime sleepiness and also make it difficult for them to fall asleep at their scheduled bedtime. Another example is jet lag, a condition that affects people who travel across multiple time zones during a relatively short period of time. Jet lag temporarily causes fatigue and disrupts sleep as the traveler’s body adjusts to the new local time.

Most circadian rhythm sleep disorders are diagnosed after patients exhibit symptoms for at least three months. While each condition above requires a specific diagnosis, many can be treated by sleep specialists with measures we’ve described above, such as light exposure therapy and melatonin supplements. Improving sleep hygiene and following a consistent bedtime schedule may also be effective. If left untreated, these disorders can have negative impacts on your physical, cognitive, professional, and social performance.