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Understanding The Circadian Rhythm

Our circadian rhythm tells us when to be asleep and when to be awake. Here's what shift workers need to know about this internal timekeeper.

Every living thing on Earth sleeps. Even bacteria have daily cycles of activity and rest. Sleep is vital, and many humans follow the same pattern, preferring to be awake during the day and asleep at night. But as long as humans have been alive, we have needed people to defy their internal clocks and stay awake at night. Long ago, sentinels protected early human camps, and today, night shift workers are vital for modern society.

Sleep is a topic that is still not fully understood because there are many complex factors at play. But at the heart of the sleep cycle are two main controls: the external clock (the sun) and our internal clock (the circadian rhythm). Unfortunately, the complex demands of our lives often conflict with these controls.

So what happens if your internal clock doesn’t line up with the external demands of your shifty schedule? Understanding the circadian rhythm can actually help you reprogram it so that you can have the two in sync with each other. This is an essential skill if you are struggling to cope with the demands and stresses of night shift work. A properly timed circadian rhythm can help improve your mood, energy, performance at work—and even your health in the long term.

But first, we need to learn a little more about it.


What is the circadian rhythm?

“Circadian” comes from the Latin “circa” meaning “about” and “dies” meaning “day.” The circadian cycle regulates a LOT:

  • Our daily levels of alertness and sleepiness
  • Our body temperature
  • Our hormones
  • Our genes


The body’s internal cycle is slightly longer than 24 hours—meanwhile, the external clock (the sun’s rotation around the earth) is 24 hours long. To keep the two in sync, the body relies on input from the environment, like light. Whether from the sun or artificial lighting, light plays a vital role in regulating your circadian rhythm.

We also receive many complex cues from the environment (called zeitgebers) that tell us when to be awake and when to sleep. These cues include eating, working, and socializing. The brain senses these cues, and in response, the body releases hormones (like melatonin and cortisol) and regulates various genes.

Both wakefulness and sleep are vital. Our awake hours are generally reserved for tasks like working, eating, socializing, and sex. On the other hand, sleep helps our bodies with immune function, metabolism, cell repair, memory consolidation, creativity, and emotional regulation.


Where does the circadian rhythm happen?

The circadian rhythm happens throughout your entire body, but it starts with the eye. Special receptors in the eye are sensitive to light, particularly short wavelength (blue) light. Instead of seeing things, these receptors sense changes in light and dark and send these messages to the brain.

Light from the environment (daylight or artificial light) enters the eye, which then sends messages to the brain’s main internal clock, the suprachiasmatic nucleus (SCN). This structure is located deep in the brain, in a location called the hypothalamus. The SCN in turn sends messages to the pineal gland, which stores melatonin. Light actually stops the release of melatonin, which tells our body that it’s time to be awake. On the other hand, a lack of light (or dark) tells our pineal gland to start releasing melatonin.



Timing of the circadian rhythm

Melatonin, the so-called sleep hormone, is released in response to dark. In most places, it gets dark in the evening, and our melatonin levels start to rise a few hours before bed. Melatonin tells the body to be sleepy and puts the body in a state of being able to sleep. Once we fall asleep, our melatonin levels eventually drop and our body’s drive to sleep goes down, allowing us to wake up in the morning. Melatonin stops being released in response to light, allowing us to feel more awake in the morning.


Most adults will naturally sleep at night for about 6–10 hours, with individual differences. There is actually a lot of diversity in how people sleep!


Diversity in the circadian rhythm

While all people have a circadian cycle, there are person-to-person differences in the specifics:

  • People with an advanced phase naturally go to bed and rise earlier. They are called morning people or larks. 
  • People with a delayed phase naturally go to bed and rise later. They are called night people or owls.
  • Many people (called everyday pigeons) will fall between these two extremes, and some people even have an irregular circadian cycle.


Circadian rhythm sleep disorders

While everyone’s circadian rhythm is unique to them, some people’s rhythms are outside of what is considered “normal.”

  • Advanced sleep phase disorder is when people have an extremely early sleep phase. They want to sleep while most people are eating dinner, and often wake up shortly after midnight.
  • Delayed sleep phase disorder is when people have an extremely delayed sleep phase. They may stay up all night, preferring to sleep in the early morning hours.
  • Shift work sleep disorder refers to the fatigue, insomnia, and difficulty functioning that shift workers may have due to their jobs.

For many people, the “disorder” really comes down to a conflict between their natural tendencies and the schedule imposed on them. These disorders can cause problems with work or school, but if left to make their own schedules, people with these phases feel perfectly fine. It’s only a disorder when your obligations conflict with what your body wants to do naturally! 

Importantly, people with a naturally delayed sleep phase may do better on night shift.

There are many other sleep disorders (like insomnia, narcolepsy, and sleep apnea) that have nothing to do with the circadian rhythm, but these can also affect your quality of sleep and can exist at the same time as a circadian rhythm disorder. Sleep is complicated!


The shift work-circadian mismatch

For most people, shift work does not come naturally. Many shift workers experience sleep deprivation, fatigue, poor performance, poor mood, and related health problems. The fundamental problem with shift work is that there is a mismatch between the internal clock and the external schedule. It’s like what happens with jet lag—your internal clock thinks it’s 3 a.m., while the outside environment tells you it’s noon. 

When a mismatch occurs for a long time, people tend to suffer. For example, shift work sleep disorder is when people who work shift work have a long-term pattern of poor sleep, sleepiness, insomnia, distress, and impaired functioning. One-third of shift workers may have this disorder.

If you don’t properly retrain your circadian rhythm to match your shifty work schedule, it’s possible that your sleep and job performance will be worse. Researchers have found that shift workers with untrained circadian rhythms experience their lowest core body temperature (Tmin) while they are still at work. In other words, their bodies are strongly signaling that it is time to sleep, making them less alert and more prone to errors and accidents. 


Our lowest body temperature: Tmin

During sleep, the body reaches its lowest core temperature for the day (called Tmin). Tmin happens for most people about 7 hours after melatonin starts to be released in the evening. Having your Tmin happen during sleep is vital for good quality sleep. 

However, many shift workers are forced to stay awake during their Tmin, when their body is at its lowest temperature and at its sleepiest. Luckily, there are ways to shift your Tmin back into your period of sleep. Therefore, understanding the concept of Tmin is especially important for shift workers who are struggling with their sleep.


An untrained circadian rhythm also means that when shift workers get home and try to sleep, they are fighting their internal clocks telling them it’s time to be awake (because it’s light out). Having a mismatched shift work schedule and circadian rhythm is like permanently having jet lag. It’s also why it’s a total myth that shift workers just need to get enough sleep during the day to ward off sleepiness on shift—even if they get enough sleep, they will still have a mismatch, which will cause them to feel tired.

So what do to about it? Shift workers have long relied on caffeine, stimulants, naps, and melatonin to get them through the long nights. While these may make short-term differences, they are not getting at the root of the problem: the workers’ circadian rhythms have not been retrained, meaning they will still be tired the next night.

Scientists have proposed several optimal ways for shift workers to reprogram their internal clocks. These methods use artificial lighting at night, darkness during the day, and light exposure in the afternoon. Reprogramming also requires having consistent sleep and wake times, not only day-to-day, but also on days off. Read Part II, to explore these ideas further and learn exactly what it looks like to retrain your circadian rhythm.

Last updated: 2024-03-09

Picture of Emma Farley, MD CCFP

Emma Farley, MD CCFP

On her journey to become a physician, Dr. Emma Farley worked many nights shifts caring for patients while balancing a heavy study workload. As a medical writer and editor at Night Shift Wellness, she creates practical and evidence-based articles for readers.

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