Circadian rhythms are cycles in the body that occur roughly across 24 hours. In humans, circadian rhythms cause physical and mental changes in the body, including feelings of wakefulness and sleep.
However, several issues may alter these circadian rhythms, which could lead to sleep disruptions or other health issues.
Keep reading to learn more, including how it works, factors that may disrupt it, and some tips on maintaining a healthful circadian rhythm.
A circadian rhythm is a natural process that takes place throughout every day. These rhythms take place everywhere, occurring throughout the natural world, such as in plants and other animals. They are essential to organisms and occur even in the absence of outside factors.
In humans, circadian rhythms are the approximate 24-hour patterns the body and brain go through, allowing for changes in the body’s physical and mental states, along with mood and behavioral changes.
The sleep-wake cycle is one of the most widely recognized circadian rhythms. Humans tend to become tired at night and feel more awake during the day. This 24-hour pattern is what most people refer to when they talk about a circadian rhythm. However, they encompass factors other than sleep.
Other examples of circadian rhythms in humans include:
- hormonal activity
- body temperature
- immune function
Circadian rhythms are vital processes that function without external factors. This is because the body itself responds to biological clocks, which exist naturally in humans and their cells.
The National Institute of General Medical Sciences note that nearly every tissue and organ contain their own biological clocks. These are the result of certain proteins interacting with cells in the body, instructing them to be more active or to slow down.
One master clock in the body controls all these individual clocks. In humans, the master clock is a structure called the suprachiasmatic nucleus (SCN), which contains about 20,000 nerve cells and receives direct input from the eyes.
As the eyes perceive the bright light of day or the darkness of night, the SCN picks up on this information, telling the cells to act accordingly. Light keeps the circadian rhythm in sync with a 24-hour day.
In addition to reactions in the cells themselves, chemicals in the brain adjust in response to the cycles of the day.
These chemicals adjust a number of factors in the body, such as:
- arousal and awakeness
How does it relate to sleep?
The body’s circadian rhythms control the sleep-wake cycle. They play a role in sleep due to how the body and brain respond to darkness, which is when most humans feel tired and tend to sleep.
As darkness sets in, the body’s biological clock instructs the cells to slow down. When the evening becomes dark, the hormone melatonin starts to rise and allows sleep to occur. Melatonin peaks around 2–4 A.M. and then reduces by morning, allowing wakefulness.
Light is the major outside factor controlling the body’s circadian rhythms. It keeps the circadian rhythm in sync with the Earth’s natural 24-hour cycle. In addition, other environmental cues may help synchronize the circadian rhythm, including food intake and activity level. However, many things can disrupt this process.
While circadian rhythms occur naturally, several factors may affect them across the day.
Irregularly-timed light can easily disrupt a normal circadian rhythm.
The Centers for Disease Control and Prevention (CDC) note that the circadian clock is most sensitive around 2 hours before a person’s usual bedtime. Using bright lights during this time can shift the need to sleep later, so a person may get sleepy and fall asleep later in the evening and wake up later in the morning.
In contrast, bright morning light can shift the need for sleep earlier. Sleeping in a bright room may also wake a person up earlier than necessary and displace their usual sleep time.
The color of lights appears to disrupt circadian patterns. The CDC note that blue wavelength light has the strongest impact.
Blue and white lights during sensitive periods of the day, such as 2 hours before bed, can make it difficult for a person to fall asleep or stay asleep. Common sources include electronic screens on devices such as phones, computers, and televisions.
Other wavelengths of light have less effect on the circadian clock.
Unhealthful sleep habits
Having unhealthful sleep habits may disrupt the circadian clock across the day. This may include issues such as:
- going out late and waking up early
- having no set sleep time
- eating and drinking late at night
- consuming caffeine late at night
- using electronic devices late at night
- performing mentally stimulating activities late in the day
- having pain or discomfort in the sleeping space
People who work late shifts or work throughout the night may experience disruptions in their natural circadian rhythms. As the body responds to the sun’s natural light and dark cycles, shift work changes their circadian rhythms.
People who travel frequently may experience disruptions in sleep and their circadian rhythms, especially if they often move between time zones. This is known as jet lag, the groggy or tired feeling as the body tries to catch up with time changes and the new rhythms of the day.
Underlying sleep disorders may affect circadian rhythms, including:
- Delayed sleep phase syndrome: When a person’s circadian rhythm becomes delayed, so they prefer to fall asleep and wake later.
- Advanced sleep phase syndrome: The circadian rhythm is advanced, so a person feels sleepy earlier in the evening and wakes earlier in the morning.
- Irregular sleep-wake disorder: There is a lack of regular rhythm that leads to sleep and waking disruptions.
- Non-24 hour sleep-wake disorder: The circadian rhythm is not synchronized to a 24-hour day, resulting in periods of sleepiness and periods of insomnia.
There are several important factors to consider when maintaining a healthful circadian rhythm.
If possible, go to bed and wake up at the same time each day. Setting a regular time may help the body set its rhythms around these times. Some choose to set a morning alarm to wake up at the same time each day. This may help the body adjust and encourage tiredness when they need to sleep to wake up on time.
This regular sleep-wake schedule also includes days off from work, such as weekends.
As light can disrupt the circadian rhythms, it is important to choose when to limit exposure. The CDC note that the 2 hours before a person falls asleep appear to be most crucial. Avoiding blue light at this time may help ensure a regular circadian rhythm, which includes limiting screen time and any bright sources of white or blue light, such as in shops.
Other tips may help promote a healthful circadian rhythm, including:
- going outside or in bright light in the morning
- avoiding caffeine late in the day
- taking small naps in the early afternoon if a person needs to nap
- avoiding long naps or napping later in the day
- avoiding heavy meals
- performing calming activities before bed, such as reading or doing gentle stretches
Some calming herbal teas or supplements may help promote a sleepy state in people with trouble falling asleep. However, talk with a doctor before taking products with active ingredients.
While it is normal to feel groggy at times, anyone who regularly experiences sleep disruptions or feels their circadian rhythms are off may want to talk with their doctor.
Poor quality sleep or sleep deprivation can lead to health complications, including:
- diabetes or insulin resistance
- sleep apnea
- heart attack
- depression and anxiety
For people with irregular schedules, such as those who frequently travel or those who work during the night, it may help to ask a healthcare professional about ways to limit circadian disruption.
Melatonin may help bring on sleep and reset the circadian rhythms, but it is important to use it correctly. Talk with a doctor before using hormones to reset a sleep cycle.
Circadian rhythms are natural cycles the body goes through each day. The rhythm of sleep and wakefulness is the most widely recognized example of these rhythms.
Maintaining a healthful circadian rhythm may involve adjusting a person’s habits to match the rhythms of nature, and may help prevent some issues with sleeping or waking.
Anyone uncertain about their symptoms should speak with a doctor for a full diagnosis and management plan.