Key Terms
Concise definitions for terms used throughout this guide
The following definitions are provided for quick reference. Full context for each appears in the relevant section of the guide.
Circadian Entrainment
The process by which the internal circadian clock is synchronised to the external 24-hour day, primarily via light exposure through ipRGCs.
Dawn Simulation
A light therapy approach using a gradually brightening light before waking, designed to mimic a natural sunrise. Has meta-analytic evidence for seasonal affective disorder comparable to standard bright light therapy.
ipRGCs (Intrinsically Photosensitive Retinal Ganglion Cells)
A class of light-sensitive retinal cells, distinct from the rods and cones used for vision, that feed light information directly to the circadian clock. Most sensitive to blue-wavelength light.
Lux
A unit of light intensity as perceived by the eye. Typical indoor lighting is roughly 300–500 lux; overcast outdoor daylight is commonly 1,000–10,000 lux; direct sunlight can exceed 100,000 lux.
Melanopsin
The light-sensitive pigment used by ipRGCs, distinct from the pigments used by rods and cones for conventional vision.
Phase Advance / Phase Delay
Shifting the circadian clock earlier (advance) or later (delay). Light exposure late in the biological night or early in the biological morning advances the clock; light exposure early in the biological night delays it.
Seasonal Affective Disorder (SAD)
A pattern of depression that recurs seasonally, typically worsening in autumn and winter. Has strong randomised trial evidence for treatment with bright light therapy.
UVB Radiation
The specific ultraviolet wavelength band responsible for triggering vitamin D synthesis in skin. Distinct from the visible light that drives circadian entrainment, and heavily affected by latitude and season.