Every living creature on Earth has evolved with an internal clock synchronised to the 24-hour cycle of day and night. In humans, this circadian rhythm governs not just when we sleep and wake, but also hormone release, body temperature, appetite, and countless other physiological processes. Understanding your circadian rhythm is key to understanding sleep—and to using tools like melatonin effectively.
What Is the Circadian Rhythm?
The term "circadian" comes from the Latin words "circa" (about) and "diem" (day), reflecting the approximately 24-hour cycle these rhythms follow. Your circadian rhythm is essentially your body's internal master clock, coordinating biological processes to occur at optimal times throughout the day.
This internal clock influences:
- Sleep-wake cycles and alertness levels
- Hormone production and release timing
- Body temperature fluctuations
- Metabolism and appetite patterns
- Immune system function
- Cognitive performance and mood
- Cell regeneration and repair processes
The 2017 Nobel Prize in Physiology or Medicine was awarded to three scientists for their discoveries of the molecular mechanisms controlling circadian rhythms. This recognition underscores just how fundamental these biological clocks are to human health.
The Master Clock: Your Suprachiasmatic Nucleus
The conductor of your circadian orchestra is a tiny region of the brain called the suprachiasmatic nucleus (SCN). Located in the hypothalamus, just above where the optic nerves cross, the SCN contains about 20,000 neurons that act as your body's central pacemaker.
The SCN receives direct input from specialised cells in your eyes that detect light—even when your eyes are closed or you're asleep. This light information allows the SCN to synchronise your internal clock with the external day-night cycle.
From the SCN, signals are sent throughout the body to coordinate the timing of various processes. One of the most important outputs is the signal to the pineal gland to produce melatonin.
Melatonin: The Messenger of Darkness
Melatonin is often called the "hormone of darkness" because its production is triggered by the absence of light. When darkness falls and light levels detected by your eyes decrease, the SCN signals the pineal gland to begin producing melatonin.
The Melatonin Cycle
In a healthy circadian rhythm:
- Evening (2-3 hours before bed): Melatonin levels begin rising as light fades
- Nighttime (2-4am): Melatonin reaches peak levels
- Morning: Light exposure suppresses melatonin, promoting wakefulness
- Daytime: Melatonin levels remain very low
Melatonin doesn't make you sleep directly—rather, it signals to your body that it's time to prepare for sleep. It lowers body temperature, reduces alertness, and creates conditions conducive to sleep onset.
Think of melatonin as the messenger that tells your body "night has arrived." It doesn't force sleep but opens the gates for sleep to occur naturally. This is why melatonin supplements work best when combined with appropriate environmental conditions—darkness, quiet, and relaxation.
The Two-Process Model of Sleep
Scientists explain sleep regulation through the "two-process model," which describes how two independent factors work together to determine when you feel sleepy and when you feel alert.
Process S: Sleep Pressure
From the moment you wake up, a chemical called adenosine begins accumulating in your brain. The longer you're awake, the more adenosine builds up, creating what's called "sleep pressure" or "sleep drive." This pressure increases throughout the day, making you progressively sleepier.
When you sleep, adenosine is cleared away, resetting the cycle. This is why a good night's sleep leaves you feeling refreshed, while sleep deprivation (even partial) leaves adenosine elevated and you feeling groggy.
Process C: Circadian Rhythm
Independent of sleep pressure, your circadian rhythm creates a daily wave of alertness and sleepiness. Typically, alertness rises after waking, dips slightly in the early afternoon (explaining the post-lunch slump), peaks in the early evening, and then falls dramatically as nighttime approaches.
How They Work Together
Sleep occurs when high sleep pressure coincides with low circadian alertness (typically at night). Problems arise when these processes are misaligned—for example, when you have high sleep pressure but high circadian alertness (trying to sleep during the day) or low sleep pressure but low circadian alertness (insomnia from napping too much).
What Disrupts Circadian Rhythms?
Modern life presents numerous challenges to our circadian systems. Understanding these disruptions is the first step to addressing them.
Artificial Light
Our ancestors' circadian rhythms were set entirely by sunlight. Today, artificial lighting—especially the blue-rich light from screens—can trick our brains into thinking it's still daytime, suppressing melatonin production and delaying sleep.
Irregular Schedules
Shift work, social jet lag (different sleep times on weekends vs. weekdays), and inconsistent routines confuse the circadian system. Your body thrives on predictability.
Travel Across Time Zones
Jet lag occurs because your internal clock is still set to your home time zone while your environment demands you operate on a different schedule.
Age-Related Changes
Circadian rhythms naturally shift with age. Teenagers tend toward later sleep times, while older adults often experience earlier sleep onset and waking. Melatonin production also decreases with age.
Long-term circadian disruption is associated with increased risks of obesity, diabetes, cardiovascular disease, depression, and even certain cancers. Protecting your circadian health isn't just about better sleep—it's about overall wellbeing.
Supporting Your Circadian Rhythm
The good news is that you can actively support healthy circadian function through daily habits and, when needed, strategic melatonin use.
Light Management
Light is the most powerful circadian cue:
- Morning: Get bright light exposure within an hour of waking
- Daytime: Spend time in natural daylight when possible
- Evening: Dim lights 2-3 hours before bed
- Night: Make your bedroom as dark as possible
Consistent Sleep Schedule
Go to bed and wake up at the same times every day, even on weekends. This consistency reinforces your circadian rhythm and improves sleep quality.
Strategic Melatonin Use
When circumstances disrupt your rhythm—travel, shift work, occasional sleeplessness—melatonin supplements can help signal to your body that it's time for sleep, even when environmental cues are confusing.
Temperature Regulation
Your body temperature naturally drops as you prepare for sleep. Supporting this by keeping your bedroom cool (16-19°C) and taking a warm bath before bed (which causes a subsequent temperature drop) can reinforce circadian sleep cues.
Chronotypes: Your Personal Rhythm
Not everyone's circadian rhythm is identical. Chronotype refers to your personal tendency toward earlier or later sleep times.
- Morning types ("larks"): Naturally wake early and feel most alert in the morning, with energy declining in the evening
- Evening types ("owls"): Naturally stay up late and struggle with early mornings, with peak alertness in the afternoon and evening
- Intermediate types: Fall somewhere between, with more flexibility in sleep timing
Your chronotype is partly genetic and partly influenced by age. Understanding your natural chronotype helps you work with your biology rather than against it.
When possible, align your schedule with your chronotype. If you're an evening type forced into early mornings, strategic melatonin use (taken earlier in the evening) can help shift your rhythm earlier. Morning types travelling west may find melatonin helpful for adjusting to later local times.
The Future of Circadian Science
Research into circadian rhythms continues to reveal new insights. Scientists are exploring "chronotherapy"—timing medical treatments to coincide with circadian patterns for maximum effectiveness. There's also growing recognition that many health recommendations should consider circadian timing.
For everyday life, the message is clear: your circadian rhythm is fundamental to your health, and supporting it through light exposure, consistent schedules, and when needed, melatonin supplementation, pays dividends in better sleep and better health.
The Bottom Line
Your circadian rhythm is a remarkable biological system that coordinates countless processes to match the 24-hour day. Melatonin is the key messenger in this system, signalling to your body when night has arrived and sleep should occur.
By understanding how your circadian rhythm works, you can make informed decisions about sleep habits, light exposure, and supplement use. When you work with your biology rather than against it, better sleep follows naturally.