GLOSSARY

Melatonin
Melatonin is a pineal-derived signal of biological night whose secretion is shaped by the central circadian system and light exposure. It contributes to temporal organization rather than simply switching sleep on.

Biological time and endocrine output

Light information helps synchronize the central clock, which coordinates pineal and wider endocrine timing through neural pathways. Peripheral clocks also respond to metabolic and behavioral cues. Clock time and internal circadian phase can diverge during travel or shift work.

A distinction that matters

Circadian phase effects depend on timing relative to the internal clock. Endogenous secretion and oral pharmacological exposure are different physiological and clinical contexts.

Timing rather than one level

A single melatonin or cortisol concentration does not describe an entire rhythm. Light exposure, sleep schedule and sampling conditions matter. Interpreting a phase shift requires a time series or an appropriately validated phase marker.