How to Reset Your Circadian Rhythm

What the circadian rhythm actually is

The circadian rhythm is a roughly 24-hour biological cycle present in nearly every cell of your body. It is not just a sleep-wake cycle — it governs cortisol release, body temperature, digestion, immune function, cell repair, and the timing of dozens of hormones.

The master clock is located in the suprachiasmatic nucleus (SCN), a tiny region of the hypothalamus containing around 20,000 neurons. The SCN receives direct light input from the retina via the retinohypothalamic tract and broadcasts timing signals to peripheral clocks throughout the body — in the liver, gut, skin, and heart.

Each of these peripheral clocks can be shifted independently by their local cues (meals for the liver, skin temperature for the skin). A disrupted circadian rhythm is not one clock going wrong — it is multiple clocks becoming desynchronised from each other and from the external environment.

What disrupts the circadian rhythm

The circadian clock is robust but not immune to disruption. Common causes include:

• Artificial light at night: Light, especially short-wavelength (blue) light from screens and LEDs, suppresses melatonin production and delays the clock. This is the most widespread source of circadian disruption in modern populations.
• Irregular sleep timing: The clock is entrained by consistent timing cues. Variable bedtimes and wake times — especially large weekend delays — weaken the clock's synchronisation and reduce sleep quality across the week.
• Jet lag and shift work: Rapid time zone changes and rotating shift work force the clock to desynchronise from the external environment. These produce measurable effects on metabolism, immune function, and cognitive performance.
• Insufficient morning light: Many people spend most of their day indoors under artificial light that is too dim to provide a strong morning entrainment signal. Outdoor light on a cloudy day is 10,000–20,000 lux; indoor lighting is typically 100–500 lux.

How melatonin fits in

Melatonin is a hormone produced by the pineal gland, triggered by darkness. It does not cause sleep directly — it signals to the body that it is dark outside. The brain interprets this as 'prepare for sleep.'

Melatonin onset (DLMO — dim-light melatonin onset) occurs roughly 2 hours before your natural sleep time. Measuring or estimating DLMO is important when using melatonin supplements for phase shifting: the supplement needs to be taken several hours before your natural DLMO to produce an advance.

Supplemental melatonin at 0.5mg — much lower than the 3–10mg found in most pharmacy products — is as effective as higher doses for phase shifting, with fewer side effects.

The role of meal timing

The liver and digestive system have their own circadian clocks, entrained primarily by meal timing rather than light. Eating at consistent times reinforces these peripheral clocks and helps them align with the SCN's light-based schedule.

Eating at irregular times or late at night — when the metabolic clock is in rest mode — is associated with impaired glucose tolerance, increased fat storage, and disrupted hormone signaling. Time-restricted eating (eating within a consistent 8–12 hour window aligned with daylight hours) has been shown in multiple studies to improve circadian alignment, independent of caloric intake.