# Light leak

> A light leak is unwanted light entering the grow space during the dark period; in flower it can delay or disrupt flowering in photoperiod cannabis.

- Canonical: https://genetixbay.com/knowledge/glossary/light-leak.html
- Also written: light leaks, light pollution, dark period interruption, night interruption, light leaking
- Glossary category: Environment and light · Evidence: Supported
- 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"

A light leak is any light reaching flowering plants during their scheduled night: gaps in tent zips, indicator [LEDs](https://genetixbay.com/knowledge/glossary/led.md) on equipment, a hallway light under the door, a streetlamp through a window. [Photoperiod](https://genetixbay.com/knowledge/glossary/photoperiod.md) cannabis measures the length of uninterrupted darkness through [phytochrome](https://genetixbay.com/knowledge/glossary/phytochrome.md), and even a brief or dim interruption can reset that clock.

## In practice

Eckels and Bugbee (2024) reported that low-level night light pollution delayed flowering in cannabis. Other short-day crops respond to remarkably low levels. A practical test: sit inside the tent with lights off for five minutes until your eyes adjust. If you can see your hand, find the leak. Cover equipment LEDs with tape, seal zip ends and use a green headlamp only for brief checks.

In veg, leaks are harmless. In flower they show as delayed [pistil](https://genetixbay.com/knowledge/glossary/pistil.md) formation, a return to vegetative growth ([re-veg](https://genetixbay.com/knowledge/glossary/re-veg.md)) or uneven ripening.

## Don't confuse with

- **Sunrise/sunset ramp**: planned dim light; it counts as day.
- **Night break**: a deliberate interruption used to hold plants in veg on short days.

## Myth check

"Light leaks cause [hermaphrodites](https://genetixbay.com/knowledge/glossary/hermaphrodite.md)" needs care. GenetixBay's house position: herms are mainly a sign of **genetics from poor breeding**, not normal cannabis. Stress such as light interruption can *reveal* a susceptible line; it does not make a stable one herm. Fix the leak, but cull plants that throw intersex flowers and don't breed from them. See [hermaphrodites](https://genetixbay.com/knowledge/diagnosis/hermaphrodites.md).

## Related terms

[12/12](https://genetixbay.com/knowledge/glossary/12-12.md), [Photoperiod](https://genetixbay.com/knowledge/glossary/photoperiod.md), [Hermaphrodite](https://genetixbay.com/knowledge/glossary/hermaphrodite.md), [13/11](https://genetixbay.com/knowledge/glossary/13-11.md)

## Read more

- [Photoperiod and light leaks: 18/6, 12/12 and the dark period](https://genetixbay.com/knowledge/environment/photoperiod-light-leaks.md)
- [Hermaphrodite cannabis (bananas and nanners): what to do](https://genetixbay.com/knowledge/diagnosis/hermaphrodites.md)

## Sources

1. Eckels M, Bugbee B, 2024. Night light pollution delays flowering in soybean and cannabis (ASHS 2024 conference abstract). https://ashs2024.sched.com/event/1dtbo/ce-4-night-light-pollution-delays-flowering-in-soybean-and-cannabis