A light cycle (or photoperiod schedule) is the programmed on/off timing of grow lights in an indoor growing environment, expressed as hours of light followed by hours of darkness (e.g., 18/6 means 18 hours on and 6 hours off). Light cycles replicate natural day length patterns and are used to control plant growth stage β vegetative growth is maintained under long days, while many flowering plants are triggered into reproductive growth by shorter day lengths.
Key Facts
- 18/6 or 20/4: standard vegetative growth cycle for most crops
- 12/12: triggers flowering in short-day plants (cannabis, some chrysanthemums)
- 16/8: common for most herbs, lettuce, and leafy greens β extends harvest window
- 24/0 (continuous light): used for some seedlings and microgreens; causes stress in some species
- Use a timer for consistency β irregular light cycles stress plants and can disrupt flowering
- Day-neutral crops (lettuce, most herbs) produce continuously regardless of light cycle length