photoperiod

Photoperiod is the length of daily light exposure in a 24-hour cycle, typically described as the interval from dawn to dusk. It varies with latitude and season, producing long days in summer and short days in winter. Because many organisms use light duration as a reliable seasonal cue, photoperiod is a major driver of timing in biology.

In animals, the retina detects light through specialized photoreceptors and relays information to the brain’s circadian clock in the suprachiasmatic nucleus. The pineal gland adjusts melatonin secretion according to night length, and the duration of melatonin signals conveys seasonal information that can regulate reproduction, migration, fur growth, diapause, and other seasonal physiology.

In plants, photoperiodism describes responses to the relative lengths of day and night. Many species flower only when day length crosses a critical threshold, so they are categorized as long-day, short-day, or day-neutral. Photoperiod also influences other developmental transitions such as leaf formation, tuber development, and bud set.

Ecological and agricultural significance: Photoperiod provides a predictable cue for seasonal timing in natural ecosystems. In agriculture, breeders and growers consider photoperiod sensitivity when selecting varieties and may use controlled lighting or shading to optimize flowering and yield in greenhouses and fields.