# 18/6

> 18/6 is the standard vegetative light schedule: 18 hours of light and 6 hours of darkness each day, keeping photoperiod cannabis from flowering.

- Canonical: https://genetixbay.com/knowledge/glossary/18-6.html
- Also written: 18-6, 18/6 light cycle, 18 on 6 off, eighteen six, 18 hours light
- Glossary category: Environment and light · Evidence: Grower practice
- Status: Draft: awaiting expert review · Updated: 2026-10-31
- Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay cannabis glossary"

18/6 means 18 hours of light followed by 6 hours of uninterrupted darkness every 24 hours. It is the most common vegetative schedule for [photoperiod](https://genetixbay.com/knowledge/glossary/photoperiod.md) cannabis: the long day keeps plants in veg, and the dark period gives a natural break that some growers use to lower room temperature and electricity costs at off-peak times.

## In practice

At 600 [µmol/m²/s](https://genetixbay.com/knowledge/glossary/ppfd.md), 18/6 delivers about 39 [mol/m²/day](https://genetixbay.com/knowledge/glossary/dli.md) (600 × 18 × 0.0036). Many growers also run [autoflowers](https://genetixbay.com/knowledge/glossary/autoflower.md) on 18/6 or 20/4; autoflowers flower on age, not day length, so the schedule there is a trade-off between DLI and power cost.

18/6 is a convention, not a biological requirement. Cannabis grows fine on [24/0](https://genetixbay.com/knowledge/glossary/24-0.md) in veg, and the GenetixBay house position endorses it. Some growers simply prefer 18/6 for cheaper power, cooler nights or a fixed maintenance window. Either works if the DLI is adequate.

[Light leaks](https://genetixbay.com/knowledge/glossary/light-leak.md) during the 6 dark hours do not matter much in veg. They matter in flower.

## Units and formula

DLI = PPFD × 18 × 0.0036 on 18/6.

## Don't confuse with

- **12/12**: the flowering schedule.
- **20/4**: a common alternative for autoflowers and veg, giving more DLI.

## Myth check

"Plants need 6 hours of sleep in veg" is a *Myth* for cannabis; see the 24/0 entry and [the photoperiod article](https://genetixbay.com/knowledge/environment/photoperiod-light-leaks.md).

## Related terms

[24/0](https://genetixbay.com/knowledge/glossary/24-0.md), [12/12](https://genetixbay.com/knowledge/glossary/12-12.md), [Photoperiod](https://genetixbay.com/knowledge/glossary/photoperiod.md), [DLI](https://genetixbay.com/knowledge/glossary/dli.md)

## Read more

- [Photoperiod and light leaks: 18/6, 12/12 and the dark period](https://genetixbay.com/knowledge/environment/photoperiod-light-leaks.md)
- [Photoperiod vs autoflower, and how autoflowering is inherited](https://genetixbay.com/knowledge/basics/photoperiod-vs-autoflower.md)

## Sources

1. Zhang M, Anderson SL, Brym ZT, Pearson BJ, 2021. Photoperiodic flowering response of essential oil, grain, and fiber hemp (Cannabis sativa L.) cultivars. Frontiers in Plant Science 12:694153. https://doi.org/10.3389/fpls.2021.694153