How Does Circadin Work in the Body
Chemical Composition, Mechanism of Action & Metabolic Effects Explained
Key Takeaways: How Circadin (Melatonin) Works
- Active substance: Melatonin, a natural hormone that regulates the sleep‑wake cycle.
- Primary action: Binds to MT1 and MT2 receptors in the brain’s suprachiasmatic nucleus.
- Physiological effect: Promotes sleep onset and helps maintain sleep (prolonged‑release formulation).
- Onset: Take 1–2 hours before bedtime; works with the body’s natural rhythm.
- Duration: Prolonged‑release tablet provides melatonin gradually over 8–10 hours.
- Indication: Short‑term treatment of primary insomnia in patients aged 55 years and older.
Circadin (melatonin) is a naturally occurring hormone that helps reset your internal body clock and improve sleep quality. This guide explains the science behind how it works.
Important Medical Advice
If you experience chest pain, fainting, severe dizziness, or signs of an allergic reaction (swelling of face/tongue, difficulty breathing), stop taking Circadin and seek immediate medical help. Do not drive if you feel drowsy. Avoid alcohol while on Circadin. Always consult your doctor before starting melatonin, especially if you have liver or kidney problems, an autoimmune disease, or are taking other medications.
Chemical Composition & Molecular Structure
The active substance in Circadin is melatonin (N‑acetyl‑5‑methoxytryptamine), a hormone naturally produced by the pineal gland. Its synthetic form is identical to the endogenous molecule.
Chemical Structure Details
N‑[2‑(5‑methoxy‑1H‑indol‑3‑yl)ethyl]acetamide
This indole‑based structure allows melatonin to cross the blood‑brain barrier and bind selectively to melatonin receptors.
C13H16N2O2
13 carbon, 16 hydrogen, 2 nitrogen, and 2 oxygen atoms.
232.28 g/mol
Low molecular weight facilitates rapid absorption and distribution.
Pharmaceutical Properties of Circadin 2 mg Prolonged‑Release Tablets
| Property | Value / Characteristic | Clinical Significance |
|---|---|---|
| Solubility | Slightly soluble in water; lipophilic | Enables passage through cell membranes and blood‑brain barrier |
| pKa | Approx. 10.8 (weak base) | Mostly unionised at physiological pH, favouring tissue penetration |
| Protein Binding | ~60% (mainly to albumin) | Moderate binding allows sufficient free fraction for receptor interaction |
| Excipients | Ammonio methacrylate copolymer, calcium hydrogen phosphate, lactose, silica, talc, magnesium stearate | Prolonged‑release matrix ensures gradual melatonin release |
🗒️ Pharmaceutical Insight: Circadin’s prolonged‑release formulation uses a pH‑dependent copolymer to delay dissolution, mimicking the body’s natural nocturnal melatonin secretion profile.
Mechanism of Action: How Melatonin Regulates Sleep
Melatonin is the key hormonal signal for darkness. It synchronises the circadian rhythm with the environmental light‑dark cycle.
Key Steps in Melatonin’s Action
- Darkness triggers production: Naturally, melatonin rises in the evening. Circadin provides an external source.
- Receptor binding: Melatonin binds to MT1 and MT2 receptors in the suprachiasmatic nucleus (SCN) of the hypothalamus.
- Neuronal inhibition (MT1): MT1 activation reduces SCN neuronal firing, promoting sleep onset.
- Phase shifting (MT2): MT2 activation advances or delays the circadian clock, reinforcing the sleep‑wake rhythm.
- Vascular effects: Melatonin also causes vasodilation in peripheral vessels, contributing to the drop in core body temperature that facilitates sleep.
🗒️ Physiological Insight: Unlike traditional sleeping pills, melatonin does not sedate the brain directly; it “opens the gate” for sleep by timing the body clock.
Melatonin Receptors in the Brain
Two high‑affinity G‑protein coupled receptors mediate melatonin’s effects:
| Receptor | Location | Function |
|---|---|---|
| MT1 | SCN, hippocampus, retina | Inhibits neuronal firing → promotes sleepiness; vasoconstriction in some vessels |
| MT2 | SCN, retina, ovaries | Phase‑shifts circadian rhythms; vasodilation in arteries |
Both receptors are also found in peripheral tissues, explaining melatonin’s roles in immunity, antioxidant defence, and reproduction.
Metabolic Effects & Duration in the Body
Melatonin is rapidly metabolised, but Circadin’s prolonged‑release design sustains levels throughout the night.
Metabolic Pathway
Primary metabolism
Liver: Mainly by CYP1A2 (and minor CYP2C19) to 6‑hydroxymelatonin, then conjugated to 6‑sulfatoxymelatonin.
Elimination
Urine: 80% as 6‑sulfatoxymelatonin; faeces: minor.
Half‑life
Immediate‑release: 30–60 min; prolonged‑release: 3.5–4 h (but with extended absorption phase).
Duration of Action
After a 2 mg dose, melatonin levels rise slowly, peak at 2–3 hours, and remain above baseline for 8–10 hours, supporting sleep maintenance without excessive morning levels.
Absorption, Distribution & Elimination
| Parameter | Value |
|---|---|
| Bioavailability | ~15% (due to extensive first‑pass metabolism) |
| Peak concentration (Cmax) | ~1.5–2 hours after intake (prolonged‑release) |
| Volume of distribution | ~1–2 L/kg – widespread tissue distribution |
| Protein binding | 60% (albumin) |
| Elimination half‑life | 3.5–4 hours (parent drug) |
🗒️ Food effect: Taking Circadin after a meal delays absorption slightly but does not reduce overall exposure. Smoking (CYP1A2 inducer) can lower melatonin levels – inform your doctor if you smoke.
Clinical Efficacy for Primary Insomnia
Circadin is indicated for short‑term treatment (up to 13 weeks) of primary insomnia in patients ≥55 years.
| Outcome | Effect vs. placebo |
|---|---|
| Sleep onset latency | Reduced by 10–15 minutes |
| Sleep quality | Significant improvement (subjective and actigraphy) |
| Morning alertness | No impairment – no “hangover” effect |
Melatonin does not cause dependence or withdrawal, making it suitable for older adults with chronic insomnia.
Circadin Mechanism FAQs
How does Circadin differ from regular melatonin tablets?
Circadin is a prolonged‑release formulation designed to release melatonin gradually over 8–10 hours, mimicking the natural overnight secretion. Standard immediate‑release tablets raise levels quickly but fall within 2–3 hours, which may not sustain sleep.
How long does Circadin take to start working?
Circadin should be taken 1–2 hours before bedtime. Melatonin levels begin to rise within 30–60 minutes, and the sleep‑promoting effect is usually felt within 1–2 hours.
Can I become dependent on Circadin?
No. Melatonin does not cause dependence or withdrawal. It regulates the body clock rather than sedating the brain, so it is not habit‑forming.
Is it safe to drink alcohol while taking Circadin?
Alcohol reduces the effectiveness of melatonin and can increase drowsiness. It is best to avoid alcohol before, during, or after taking Circadin.
What happens if I take more than 2 mg?
Taking more than the recommended dose may cause drowsiness, headache, or dizziness. If you accidentally take too much, contact your doctor or pharmacist.
Need Help with Jet Lag or Poor Sleep?
If you're 55 or over and struggling with sleep onset or quality, Circadin may help reset your sleep pattern. A UK‑registered doctor can advise if melatonin is suitable for you.
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Clinical References & Sources
Medical Content Manager
Nabeel is a co-founder, and medical content manager of Chemist Doctor. He works closely with our medical team to ensure the information is accurate and up-to-date.
Medical Doctor
Dr. Feroz is a GMC-registered doctor and a medical reviewer at Chemist Doctor. He oversees acute condition and urgent care guidance.
Medical Director
Usman is a co-founder, and medical director of Chemist Doctor. He leads the organisation's strategic vision, bridging clinical and operational priorities.
Review Date: 16 February 2026
Next Review: 16 August 2026
Published on: 16 February 2026
Last Updated: 16 February 2026