Circadian Rhythm and Your Body Clock
All living beings have an internal body clock that develops patterns in response to environmental factors like light and temperature. Known as circadian rhythms, these patterns are responsible for many of our bodily functions, including sleep.
What Is Circadian Rhythm?
Circadian rhythms are internal, 24-hour cycles that regulate physiological and psychological processes. In humans, the timing of these circadian rhythms is mostly determined in response to sunlight, hence the name circadian, meaning “about a day”. These cycles affect a variety of physical and mental operations, from blood pressure and mood to physical strength and speed. While circadian rhythms repeat approximately every 24 hours, they can also adapt to changes in the individual's surroundings.
Circadian rhythms respond to environmental cues called zeitgebers. In addition to light and heat, common zeitgebers include social interactions, meal schedules, and work schedules. However, studies show that circadian rhythms retain a certain level of regularity even without the presence of zeitgebers, due to a series of biological clocks that coordinate these internal processes.
What Are Body Clocks?
The circadian rhythm in humans is controlled by multiple biological clocks that work together under the guidance of a master clock.
Biological clocks are proteins within the body that regulate circadian rhythms for different systems, such as appetite, body temperature, cognitive functioning, and sleep-wake patterns. Animals, plants, and bacteria all use biological clocks to align and synchronize the rhythms that are required for survival. Without biological clocks telling it when to sleep, wake, and eat, an organism would be more likely to face predators, starvation, and health problems.
Internal processes in individual biological clocks help establish a basic circadian rhythm. These clocks are also influenced by external cues and synchronized by a master clock in the brain.
The Master Clock
A part of the brain called the suprachiasmatic nucleus (SCN) plays the role of master clock in humans. Located in the brain's hypothalamus, the SCN coordinates all of the biological clocks within an organism. The SCN receives signals directly from the eyes, making it highly sensitive to changes in light.
How Do Circadian Rhythms Affect Sleep?
As part of the sleep-wake cycle, the SCN uses environmental cues to tell the body when to feel tired and when to wake up. The circadian rhythm works in tandem with a function called the homeostatic sleep drive to induce sleepiness at the correct time of day.
When the eye is exposed to light, the SCN releases chemicals and hormones that promote alertness. In dim light or darkness, the brain responds by telling the pineal gland to secrete the sleep-inducing hormone melatonin. The SCN must receive the proper amount of light or darkness at the correct time of day to maintain a regular sleep-wake cycle.
What Happens When Your Circadian Rhythm Is Disrupted?
A circadian rhythm that falls out of alignment with a person’s desired sleep-wake schedule can cause daytime sleepiness and trouble sleeping at night, which may lead to reduced sleep overall. In the long term, disrupted circadian rhythms are directly and indirectly linked to a variety of physical and mental health problems, including:
- Some forms of cancer
- Reduced immune system functioning
- Heart disease
- Seasonal affective disorder and depression
- Increased stress
- Poor work or academic performance
- Increased risk of injuries and accidents
- Attention and memory problems
- Sleep problems
Disruptions to the sleep-wake cycle may contribute to a circadian rhythm disorder. For example, people with blindness may lack neural pathways that interpret light cues and often develop non-24-hour sleep-wake rhythm disorder, in which the circadian rhythm does not match the typical 24-hour cycle.
Other circadian rhythm disorders include advanced sleep-wake phase disorder, delayed sleep-wake phase disorder, shift work disorder, jet lag disorder, and irregular sleep-wake rhythm disorder. These affect a person’s ability to sleep or be alert when needed.
What Can Disrupt Your Circadian Rhythm?
Changes to your sleep schedule can affect your circadian rhythm, especially if they happen consistently over an extended period of time. Other factors that impact the circadian rhythm include:
- Evening exposure to blue light from devices like cell phones and tablets
- Shift work
- Mood disorders
- Traveling across time zones
- Neurological disorders and brain damage
How To Maintain a Healthy Circadian Rhythm
Good sleep hygiene practices can help you shape your environment in a way that encourages a healthy circadian rhythm:
- Keep a Consistent Bedtime Schedule: Going to bed at the same time each night can help you feel sleepy at bedtime.
- Avoid Blue Light in the Evening: Try putting your device on dark mode, or wear special glasses that block blue light if you must be on your laptop, tablet, or cell phone late at night.
- Exercise During the Day: Some evidence indicates that daytime exercise may help regulate the circadian rhythm and help to ward off some of the metabolic problems associated with circadian rhythm disorders.
- Reduce Alcohol, Caffeine, and Nicotine: These substances interfere with sleep, so it is best to avoid them in the leadup to bedtime.
Talk to a doctor if you feel tired during the daytime or experience problems sleeping soundly at night. If you are a shift worker or plan to travel across time zones, ask your doctor about other methods to prevent jet lag or sleep disruption.
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