GLOSSARY

Melatonin and Sleep Timing
Melatonin signals biological night and can influence sleep timing through circadian mechanisms.

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.

Interpreting the mechanism

• Timing, light and individual phase matter more than assuming one universal sleep-onset effect.

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.