The pineal gland and melatonin !
- Regulation
- For completeness
In darkness the pineal gland releases melatonin, a signal that tells the body it is night. Light, especially blue light, switches it off.
Updated 2026-11-01
Also called: Epiphysis, Circadian rhythm, Suprachiasmatic nucleus
See it in 3D: Pineal gland · Hypothalamus
What it is
The pineal gland is a small gland in the roof of the third ventricle that turns the light–dark cycle into a hormonal signal. Its cells, pinealocytes, make melatonin from serotonin at night under sympathetic control, and light at the eyes suppresses it. Melatonin helps synchronize the circadian clock in the suprachiasmatic nucleus and sleep onset; in humans its role in reproduction is minor. It is a timing signal, not a sedative in the usual sense.
How it works, step by step
- Darkness: retinal ganglion cells with melanopsin stop firing to the SCN.
- The SCN releases its inhibition of the paraventricular nucleus, which drives preganglionic sympathetic neurons in the upper thoracic cord.
- Postganglionic fibres from the superior cervical ganglion release norepinephrine onto pinealocytes.
- β1 receptors raise cAMP, which activates and stabilizes AANAT, the rhythm-setting enzyme.
- Melatonin is released straight into blood and CSF; it is not stored.
- It acts on MT1 and MT2 receptors in the SCN to reinforce the night phase and lower core temperature.
- The liver clears it (half-life under an hour); the urinary metabolite 6-sulfatoxymelatonin reflects nightly output.
Regulation
- +Darkness via sympathetic norepinephrine (β1): Raises AANAT and melatonin synthesis.
- −Light at the retina (blue wavelengths): Suppresses secretion and shifts the clock.
- −β blockers: Lower nighttime melatonin.
- −Ageing and pineal calcification: Smaller nocturnal peak.
Clinical notes
- Jet lag and shift work: the clock and the light schedule are misaligned; the rhythm shifts with the timing of light exposure.
- Spinal cord injury above T1 or cervical sympathetic damage can abolish the melatonin rhythm.
- Pineal germinomas in boys can secrete hCG and cause precocious puberty.
- Pineal calcification is common in adults and serves as a midline landmark on imaging.
Test yourself
Think of the answer, then tap the question to see it.
Why does a lesion of the cervical sympathetic chain abolish the melatonin rhythm even though the eyes and SCN are intact?
The SCN reaches the pineal only through a sympathetic relay (spinal cord → superior cervical ganglion). Without norepinephrine, pinealocytes cannot raise AANAT at night.
Why does screen use before bed delay sleep onset?
Blue-rich light activates melanopsin ganglion cells, which signal the SCN to suppress pineal melatonin and shift the clock later.
Why can a pineal tumour cause upward gaze palsy?
The pineal sits above the dorsal midbrain; a mass compresses the vertical gaze centre in the tectum (Parinaud syndrome) and can block the cerebral aqueduct.
See it in 3D
The structures of Durgical's 3D anatomy where it happens.
Related topics
- Wakefulness, sleep, the EEG and seizures
- Functions of the hypothalamus
- Puberty
- Body temperature regulation
- The HPA axis and its circadian rhythm
In Biochemistry
In Pharmacology
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: 17.7 The Pineal Gland (OpenStax (Rice University), 2022); Physiology, Pineal Gland (StatPearls (NCBI Bookshelf), 2024); Serotonin and melatonin biosynthesis (Reactome, 2024); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 81). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).
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Representative model, for educational purposes only.