Circadian Rhythm: How Your Body Clock Runs Your Whole Day

It's 12:40 at night. You planned to sleep by eleven, but one short video led to another, and suddenly, almost two hours have passed. Your eyes feel tired, yet your mind is still wide awake. Tomorrow's alarm is set for 6:30 am, and you already know how difficult getting out of bed will feel. There's a reason your body feels this way. Deep inside, you have a natural clock called your circadian rhythm, which follows a roughly 24-hour cycle. It helps your body know when to feel awake, when to wind down and when it's time to sleep. Staying up late, especially with bright screens, can disrupt these signals. And sleep is only part of the story.

Your circadian rhythm also influences your appetite, digestion, body temperature, hormones and ability to concentrate. When your daily routine works against this internal clock, you may notice the effects in more ways than just feeling sleepy the next morning. The good news is that small changes to your everyday habits can help keep your body clock on track. Understanding how it works is the first step towards better sleep, steadier energy and feeling more alert throughout the day.

What is a circadian rhythm?

The word comes from Latin for "about a day". The US National Institute of General Medical Sciences describes circadian rhythms as including some of the physical, mental and behavioural changes an organism goes through over a 24-hour cycle. Plants have them, animals have them and so do many microorganisms. In people, a master clock sits deep in the brain. It is a cluster of nerve cells called the suprachiasmatic nucleus, or SCN, which rests just above the point where the nerves from your two eyes cross. The NIGMS explains that the SCN controls the hormone melatonin based on the amount of light the eyes receive. Melatonin is the hormone that tells the body night has come. But the brain is not the only clock. Almost every organ, from the liver to the gut to the muscles, keeps its own time. The master clock acts like a conductor, keeping all of these smaller clocks in step.

A clock built from genes

How a body keeps time was a mystery for a long while. In 2017 the Nobel Prize in Physiology or Medicine went to Jeffrey Hall, Michael Rosbash and Michael Young for discovering the molecular mechanisms that control the circadian rhythm. Working in fruit flies, they showed how a set of genes switch on and off in a loop that takes about 24 hours to complete. Each cell carries a tiny version of this clock. Emerging evidence from animal and human research suggests these clock genes are part of how cells regulate their metabolism. That is one reason a body clock that keeps getting disturbed is being studied as a risk factor for obesity, metabolic syndrome and type 2 diabetes.

How does your body clock run your day?

Picture an ordinary weekday when the clock is working well. Morning light hits your eyes as you open the curtains or step onto the balcony. The NIGMS notes that light and dark have the biggest influence on circadian rhythms. It resets the clock for the day and tells it the night is over. Melatonin falls away and you start to feel alert. Through the day, your organs follow the schedule the clock has set. Your gut and liver expect food during daylight hours. Body temperature climbs slowly.

As evening light fades, the SCN lets melatonin rise. Your body temperature begins to dip. Sleepiness builds. If you go to bed at a regular time, you drift off with less effort. Through the night the body repairs, files away memories and prepares to do it all again.

Also read: How Much Deep Sleep Do Adults Need? Understanding What Happens While You Sleep.

What disrupts your circadian rhythm?

Your body clock works best when it follows a regular pattern. However, everyday habits such as late-night scrolling, evening coffee and changing sleep schedules can interfere with it. Here are some common reasons your circadian rhythm may get disrupted.

Using your phone late at night

Scrolling through your phone before bed might seem relaxing, but the light from your screen can make it harder for your body to prepare for sleep. In a small Harvard study, researchers compared people reading on a light-emitting e-reader with those reading a printed book before bedtime. People using the screen took longer to fall asleep, produced less melatonin (the hormone that helps regulate sleep), experienced a delay in their body clock and felt less alert the next morning. One reason is the blue-rich light emitted by screens, including smartphones and tablets, which can interfere with your body's natural sleep signals.

Drinking chai or coffee too late

That evening cup of chai or filter coffee might be affecting more than just how sleepy you feel. A small study found that consuming caffeine equivalent to a double espresso three hours before bedtime delayed the body's melatonin rhythm by approximately 40 minutes. Caffeine can therefore affect both your ability to fall asleep and the timing of your internal clock. If you struggle to sleep at night, consider moving your last cup of tea or coffee earlier in the day.

Sleeping late on weekends

Do you stay up late on Friday and Saturday, sleep in on Sunday and then struggle to wake up on Monday? This difference between your natural sleep schedule and your work or social routine is called social jetlag. A large study found an association between greater social jetlag and higher body mass index (BMI), even after accounting for sleep duration. However, this doesn't establish that irregular sleep directly causes weight gain. Think of it as giving your body a different time zone every weekend, even though you haven't travelled anywhere.

Eating late and working night shifts

Your digestive system also follows a daily rhythm. Your gut, liver and other organs respond to signals from both your body clock and the timing of your meals. Regularly eating very late can create a mismatch between these signals, while night shifts can make it difficult to maintain a consistent sleep and eating schedule. If your working hours change frequently, keeping meals and sleep as regular as possible within your schedule may help.

Also read: Best Time to Eat: How Meal Timing Affects Digestion and Energy

Are you naturally an early bird or a night owl?

Some people wake up feeling energetic at sunrise, while others feel more alert and productive in the evening. This natural preference is called your chronotype. Your chronotype is influenced by several factors, including genetics, age and exposure to light. It also changes throughout life. Teenagers, for example, often prefer later sleep and wake times than older adults.

Being a night owl doesn't automatically mean your circadian rhythm is unhealthy. Similarly, waking up at 5 am isn't necessarily better for everyone. What matters most is maintaining a reasonably consistent routine that allows you to get enough sleep. You may not be able to completely change your natural preference, but regular sleep times and exposure to daylight can help keep your body clock aligned.

How to reset your body clock

An international group of sleep and light scientists published simple guidance in 2022. Their advice comes down to bright days, dim evenings and dark nights, with evening light kept low for at least three hours before bed. Here is how that looks in an Indian home.

1. Get daylight soon after waking. A few minutes on the balcony, the terrace or a walk to the market counts.

2. Wake up at roughly the same time every day, weekends included. A steady wake time gives your clock the same morning signal every day.

3. Work near a window during the day if you can.

4. After dinner, switch to warm, dim lamps instead of bright white tube lights.

5. Put the phone away, or at least turn the brightness down, well before bed.

6. Have your last cup of tea or coffee earlier in the day. The caffeine study above gave its dose three hours before bed.

7. Avoid eating right before you sleep where possible.

8. Keep the bedroom dark. Cover bright standby lights and use curtains that block street light.

Keep the routine steady, weekends included. Give your clock time to settle.

What about melatonin tablets?

Melatonin is a hormone that helps regulate your sleep-wake cycle. It's also available as a supplement, sometimes used for jet lag or sleep problems related to shift work. However, when you take melatonin and how much you take both matter. Speak to your doctor before using it, especially if you're pregnant, take regular medicines or have an existing health condition.

Also read: Melatonin vs Magnesium: Which Is Better for Sleep?

When should you see a doctor?

The steps above help many people. See a doctor if:

• You lie awake most nights and changing your habits has not helped

• You snore loudly, gasp or stop breathing in your sleep

• You fall asleep while driving, in meetings or mid-conversation

• You wake through the night again and again, a pattern our guide to waking up at night explores

• Your mood stays low or anxious along with poor sleep

Back to twenty to one

Tonight, the phone can go face down a little earlier. Tomorrow, open the curtains and stand in the light for a few minutes before you check your messages. Your clock has been keeping time for you all your life. Give it clear mornings and quiet evenings. It will start to feel like it is on your side again.

Sources

• Circadian rhythms fact sheet (US National Institute of General Medical Sciences)

• The Nobel Prize in Physiology or Medicine 2017: press release (Nobel Prize Outreach)

• Circadian disruption, clock genes and metabolic health (J Clin Invest, 2024)

• Evening use of light-emitting eReaders negatively affects sleep, circadian timing and next-morning alertness (PNAS, 2015)

• Effects of caffeine on the human circadian clock in vivo and in vitro (Sci Transl Med, 2015)

• Social jetlag and obesity (Curr Biol, 2012)

• Recommendations for daytime, evening and nighttime indoor light exposure (PLoS Biol, 2022)

Medical Disclaimer: This article is for general information and is not a substitute for advice from your doctor. Speak to a qualified healthcare professional before starting any supplement, especially if you are pregnant, breastfeeding or taking medication.