Definition:
The suprachiasmatic nucleus (SCN) is a small region of the brain located in the hypothalamus, directly above the optic chiasm. It is considered the body’s master circadian clock, synchronising internal rhythms with the external light–dark cycle.
Why it matters:
The SCN receives direct input from the eye via ipRGCs (intrinsically photosensitive retinal ganglion cells). This makes it the primary site where light information is converted into biological time signals. The SCN regulates circadian rhythms in hormone secretion (such as melatonin and cortisol), sleep–wake cycles, body temperature, and alertness.
How it works:
- Light detected by ipRGCs reaches the SCN through the retinohypothalamic tract.
- The SCN synchronises peripheral clocks throughout the body.
- Signals from the SCN influence pineal melatonin release, shifting sleep timing and circadian phase.
Relation to light measurement:
Metrics such as melanopic EDI and circadian stimulus are designed to quantify how light at the eye stimulates the pathways that ultimately influence the SCN. Measuring these quantities with spectral light dosimeters helps researchers and designers understand how environments affect circadian entrainment.
Applications:
- Research in circadian biology and chronobiology.
- Clinical and healthcare settings, e.g. managing circadian rhythm disorders.
- Lighting design for workplaces, schools, hospitals and aged care facilities.
- Smart building systems aiming to align light schedules with circadian health.
Related terms: