Circadian Rhythm and Our Biological Clock

What Is Circadian Rhythm?

Your body’s circadian rhythm is what regulates internal changes within your brain and body over the course of a day, including sleep and wakefulness.

It is believed that organisms with circadian clocks gain an evolutionary advantage through extrinsic adaptive value (synchronizing behavior and physiological processes to cyclic environmental factors) and intrinsic adaptive value (coordinating internal metabolic processes).

Circadian is derived from the Latin word circa (“around” or “approximately”) and diem (“day”).

These rhythms are endogenous or “built-in” to our bodies. Still, they can also be influenced by outside stimuli, meaning that they are adjusted to the external environment through cues such as light and temperature.

The Body’s “Master Clock”

The master clock coordinates all the biological clocks of a living organism. In humans, the master clock is the Suprachiasmatic Nucleus (SCN), a small cluster of around 20,000 nerve cells located within the hypothalamus.

The SCN reacts to light, the predominant zeitgeber, or cue that entrains the circadian rhythm to synchronize with the light-dark cycle.

The SCN receives light signals from the eye. The light signals then travel from the hypothalamus to the pineal gland, which regulates the release of the hormones that make us sleepy or awake.

How Does the Circadian Rhythm Affect Our Sleep?

The change in levels of the sleep hormone melatonin during the sleep-wake cycle reflect circadian rhythms.

Exposure to light during the day triggers the production of hormones that promote wakefulness. As the light fades and disappears in the evening, this triggers the production of melatonin, a sleep-promoting hormone.

During the night, the body’s master clock continues sending signals that tell the body it is nighttime and continue sleeping.

Seasonal changes can affect our body’s circadian rhythm, impacting our sleep and level of tiredness — we may feel tired more often during the peak of winter when there’s less natural light, and the sun goes down earlier.

Is Circadian Rhythm the Same as a Biological Clock?

No. Circadian rhythms are produced and regulated by an organism’s biological clock. So, they are closely related, but not the same thing.

So, What is the Biological Clock in humans?

A biological clock is a collection of cells or molecules that regulate and coordinate the rhythm and timing of biological functions.

Sleep-Wake Homeostasis

Circadian rhythms and homeostasis are two interconnected mechanisms that your body utilizes to regulate sleep and wakefulness.

An easy way to distinguish the two is to look at them like this: homeostasis tracks your body’s need for sleep, and your body’s circadian rhythm is responsible for the timing of sleep.

It is sleep-wake homeostasis that tells us it is time for sleep — it’s what creates the sleep drive in humans.

Quite simply, it works like this: the longer we are awake, the stronger the desire or need for sleep becomes, and the longer we’ve been asleep, the more the need or pressure for sleep dissipates, meaning we’re more likely to wake up.

But it doesn’t quite work like that — if it were sleep-wake homeostasis on its own that dictated sleep, we would simply get more and more tired the longer we’re awake, and we’d likely nap in cycles during the day as the drive for sleep would build until we succumbed, then dissipate as we napped.

And, while as a general rule, sleep pressure builds the longer we are awake, it is our circadian rhythm that dictates how sleepy or awake we feel throughout the day — we have peaks and troughs of wakefulness and tiredness throughout the day.

The circadian rhythm allows us to overcome the drive for sleep and stay awake continuously for around 16 hours out of every day.

How to Maintain a Healthy Circadian Rhythm

While our circadian rhythm is out of our full control, maintaining healthy sleep habits is the best way to ensure it stays on track.

Following these guidelines will also allow you to reset your circadian rhythm if it does get disrupted:

How Does Circadian Rhythm Develop and Change With Age?

Our circadian rhythm is not constant throughout our life. It develops and undergoes changes with age, from birth into old age, and is partly responsible for the vastly different sleeping habits among humans in different life stages.

1. Babies & Children

One way to think about the circadian rhythm in babies is that they come with the clock; it’s just not pre-programmed.

Even though they require 14-17 hours of sleep per day, newborn babies will not sleep in one block as adults do. Their sleep is sporadic and comes in a series of naps spread over 24 hours.

Babies sleep this way because their circadian rhythm has not yet become entrained — entrainment is the process that syncs the circadian rhythm with the light-dark cycle.

It takes until around three or four months of age for the circadian rhythm to become entrained in babies.

As children get older, the need for naps throughout the day diminishes, and their sleep begins to resemble that of adults, with sleep mirroring that of adults (i.e., one sleep block at night) around the ages of five to seven.

For everything you need to know about sleep in babies and children, you can read our Parents’ Guide to Children’s Sleep here.

2. Seniors

Our circadian rhythm changes as we age as adults, too.

As we age, we can become prone to desynchronization of our circadian rhythm, resulting in going to bed earlier and getting up earlier in the morning (phase advance). Sleep patterns can become irregular, and the requirement for daytime naps can often return.

Neurological changes may be to blame for our sleep changing as we age — our brain produces less growth hormone (which may increase deep sleep) and melatonin as we age.

Another factor that may account for changing circadian rhythms and sleep patterns as we age is changes in our daily activities. For example, post-retirement, often people’s days will suddenly become unstructured, meaning new routines and activities.

For more on seniors and sleep and how to ensure safe sleep as we age, see our guide on sleep and aging here.

What Can Disrupt Our Circadian Rhythm?

There are external factors that are prevalent in modern life that can disrupt our natural circadian rhythm and sleep cycle:

• Irregular and inconsistent mealtimes.
• Exposure to light within an hour of bedtime (blue light from device screens is especially bad).
• Light sources in the bedroom (lights from electronics, alarm clocks, streetlights, and other external lights).
• Not being exposed to bright light upon waking in the morning.

On top of these everyday factors that are usually within our control, several circadian rhythm-related disorders can have an adverse effect, too:

1. Jet Lag

If you’ve ever flown long-haul, then you may well have paid the price in the form of jet lag disrupting your sleep.

Jet lag is caused by crossing multiple time zones in a short period. Your body’s circadian rhythm is still synced to the time zone you left and is not yet synced to the time zone you ended up in. The more time zones you’ve crossed, the more out of sync your rhythm will be.

Luckily jet lag is temporary, and regular sleep will come with acclimatization to the new time zone.

2. Shift Work Sleep Disorder

Shift work, by its very nature, forces people into unnatural sleep patterns. Shift Work Sleep Disorder (SWSD) affects people who work nontraditional or irregular hours that overlap with normal sleep time.

SWSD is characterized by insomnia and excessive sleepiness, and as a result, it can have long-term health consequences.

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3. Advanced Sleep Phase Syndrome

Advanced Sleep Phase Disorder (ASPS) is a disorder where the sufferer’s circadian rhythm causes them to fall asleep earlier and wake up earlier than would be considered normal.

The cause is in the name — the internal sleep clock is out of phase with and advanced compared to clock time.

Sleep duration and quality will typically be the same; the body clock and circadian rhythm are just ahead of where they should be.

4. Delayed Sleep Phase Syndrome

Delayed Sleep Phase Syndrome (DSPS) is essentially the opposite of Advanced Sleep Phase Syndrome. It is characterized by a shift in the body’s internal clock and circadian rhythm that causes sufferers to go to sleep hours later and wake up hours later than is conventional or acceptable.

With both ASPS and DSPS, the body’s circadian rhythm is out of sync with external stimuli.

5. Non-24 Hour Sleep-Wake Rhythm Disorder

Non-24 hour sleep-wake rhythm disorder (Non-24) is a disorder common in, but not exclusive to, blind people. Non-24 is common in blind people because it is characterized by the body’s internal clock not synching with the 24-hour light-dark cycle.

Light not reaching the Suprachiasmatic Nucleus (SCN), which acts as one of the most significant external cues for generating the body’s circadian rhythm, is the reason why this disorder is so common in blind people.

6. Irregular Sleep-Wake Rhythm Disorder

Irregular Sleep-Wake Rhythm Disorder (ISWRD) is a circadian sleep disorder characterized by the absence of a circadian pattern in the sleep-wake cycle.

Sufferers of this disorder will still get the same total amount of sleep in a day, but without a “main” sleep episode, sleep happens in an irregular napping pattern across a 24-hour period.

ISWRD is usually caused by neurological disorders like dementia, brain damage, or intellectual disability.

The Effects of a Disrupted Circadian Rhythm

There are health implications due to a disrupted circadian rhythm.

Chronic sleep deprivation can be one of the consequences of a disrupted circadian rhythm, and there are potentially serious, even life-shortening adverse health outcomes from a regular lack of sleep, including:

• Adverse mental health effects — low mood, depression, anxiety, etc.
• Immune deficiency
• Hormone imbalances
• Increased wrinkles, decreased skin elasticity, more prone to breakouts, and generally poor skin.
• Increased appetite and junk food cravings, weight gain
• Diminished memory, concentration, focus, alertness, and ability to learn
• Diminished problem-solving skills, increased reaction time.
• Increased risk of:
  • Raised blood pressure
  • Cardiovascular disease
  • Obesity
  • Type 2 diabetes
  • Chronic inflammation
  • Low testosterone and low sex drive