Most people feel tired at night and hungry at lunchtime. That’s because our bodies have a built-in process that regulates sleep, hunger, and energy levels -- the circadian rhythm. Although the cycle can be affected by artificial lights or travel, unless a health issue is at play, our 24-hour "clock" always resets itself in time. We share this rhythm with all life on earth, from plants to animals, and even bacteria, and it affects more than just sleep and hunger.

What is a Circadian Rhythm?

Sometimes called the body clock, the circadian rhythm is about 24 hours long. All life, even the simplest cyanobacteria, has a circadian rhythm of the same duration. This daily cycle influences brain-wave activity patterns, cell repair, and hormone levels. The cycle is built into to all life, but changes in light levels and temperature conditions can alter them, if temporarily.

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Why We Have Circadian Rhythms

Scientists know all life has a circadian rhythm of about 24 hours, but no one knows why. An educated guess suggests the cycle is linked to the day length. However, the answer may not be this simple, as the earth has not always had a 24-hour day. 400 million years ago in the Cambrian, when animals evolved onto land, the day was about 21 hours long. The most popular theory today is that circadian rhythms are linked to changing oxygen levels in the air. Hours of sunlight affect this factor because plants only release oxygen in daylight. Scientists do not understand the exact reasons for this link.

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Discovery of the Circadian Rhythm

A long time ago, people noticed humans, plants, and animals have daily cycles of hunger and sleep. The first known written record is from a 13th-century Chinese medical textbook. In the year 1729, a French scientist Jean-Jacques d'Ortous de Mairan recorded observations about circadian rhythm as he watched a plant "wake" and "sleep" over 24 hours, even when kept in darkness. It wasn’t until 1977 that the scientific community agreed on the term we use today and exactly what this meant. Joseph Takahashi first discovery gene coding for the circadian rhythm in 1994 by studying the DNA of mice.

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The Wide Reach of the Circadian Rhythm

The circadian clock affects more than when we feel sleepy. So far, scientists have discovered 35 conditions it affects:

  • Blood pressure is lowest at three am, during deep sleep, and highest when we wake up, meaning risks for heart attacks and stroke are highest between 8 am and midday.
  • Asthma attacks are worse during daylight because the body creates a natural anti-inflammatory during night time.
  • Cells grow and repair more quickly during daylight hours.

Research into circadian rhythms has led to a new field of medicine, chronotherapy; doctors use this knowledge to administer medicine at the times it will be most effective.

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The Brain Controls the Circadian Cycle

A section of the brain called the hypothalamus controls the circadian rhythm of animals. At night the brain releases melatonin, which makes you feel tired. Although the circadian rhythm is built-in and can work without exposure to daylight, it can be altered by outside conditions. If you have ever experienced jet-lag, you know what it feels like when the circadian rhythm tries to acclimatize to new conditions. Travel affects the circadian rhythm because darkness encourages the brain to release the sleep hormone melatonin, while daylight releases cortisol to wake the brain.

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Your Rhythm Might be Different than Others

While all humans have a circadian rhythm of 24 hours, your alignment is probably different than your friends'. People who feel energized later in the day are sometimes called night owls. Their built-in circadian rhythm is different from that of "morning people." The technical term for whether someone is a morning or night person is their chronotype, and it seems to be genetic. If you are a night owl, then likely people in your immediate family are, too. Your chronotype might alter with your age, but it is rare for someone to completely change from a morning person to a night owl, or vice versa.

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Circadian Rhythm Alters with Age

While you cannot change your chronotype, your peak energy time will alter with age, which is why parents and their children differ in their most energetic hours. On average, young children experience high energy in the early morning, while teenagers peak in the afternoon. A common complaint from parents is how difficult it is to get teenagers up in the morning, but this isn’t just laziness – it’s their circadian rhythm. Some schools have even changed their schedules to allow teenage students to attend later in the day, which can improve information retention and learning. During adulthood, the circadian rhythm settles into your chronotype.

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Circadian Rhythm Disorders

In medical science, abnormal circadian cycles fall under the category of circadian rhythm disorders (CRD). These are separated into two groups based on what causes the condition. External changes cause extrinsic CRDs. For example, a person who works the night shift is not getting much daylight, and this will affect her circadian rhythm. Intrinsic CRDs are more serious because the problem is internal. Emotional, hormonal, or genetic complications can cause these disorders.

The conditions linked to CRD include

  • Attention Deficit Hyperactivity Disorder (ADHD)
  • Heart disease
  • Cancer
  • Metabolic syndrome
  • Insulin resistance (diabetes)
  • Cognitive disfunction
  • Obesity

circadian rhythm disorders South_agency / Getty Images


Impact of Modern Technology

The blue light emitted by mobile phones, TVs, and tablets mimics the blue we see in the sky during the daytime. Our bodies have evolved to be awake during the day, so our brains release the hormone cortisol when we see blue-ish light. Cortisol wakes up the brain, making us alert and active. However, if you are browsing the internet or watching TV before bed, the brain cannot distinguish between the natural blue of daylight and the artificial blue of screens. Looking at screens up to an hour before bedtime can increase cortisol levels and reduce the effectiveness of the sleep hormone melatonin on your brain. As a result, we may have difficulty sleeping or experience lower-quality sleep, leaving us tired the next day.

modern technology and circadian rhythm sam thomas / Getty Images


The Importance of Sleep Hygiene

Sleep hygiene describes good habits that improve the quality of sleep. The stress of modern life and the use of electronic devices before bed means more people are waking up feeling tired; their circadian rhythms are struggling to cope with the unnatural sequence of our modern lives. Furthermore, working indoors with little exposure to natural light tricks the body into thinking it is nighttime. Over time, exposure to these factors can create circadian rhythm disorders and larger health issues.

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