# What happens if my lights accidentally stay on during flower?

> One accidental 24-hour light period in flower usually costs a photoperiod plant a day or two, not the crop. Fix the timer and return to your normal 12/12 or 13/11 straight away; no long 'make-up' darkness is needed. It matters most in the first two weeks after the flip, when induction can stall. Watch for pollen sacs for 2–3 weeks: a plant that herms from one night is herm-prone.

- Canonical: https://genetixbay.com/knowledge/answers/lights-stayed-on-during-flower.html
- Topic: Environment · Type: Troubleshooting
- Evidence: Grower practice · Status: Draft: awaiting expert review
- Full article: [Photoperiod and light leaks: 18/6, 12/12 and the dark period](https://genetixbay.com/knowledge/environment/photoperiod-light-leaks.md)
- Also asked as: My timer failed in flower: what now? | Lights were on for 24 hours by mistake | I missed a dark period in flower | Will one night of light cause hermies?
- Updated: 2026-10-31 · Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay Knowledge Base (genetixbay.com/knowledge)"

## Short answer

One missed dark period in flower usually costs a [photoperiod plant](https://genetixbay.com/knowledge/glossary/photoperiod.md) **a day or two**, not the crop. Fix the timer and go straight back to your normal **[12/12](https://genetixbay.com/knowledge/glossary/12-12.md) or [13/11](https://genetixbay.com/knowledge/glossary/13-11.md)**; no long make-up darkness is needed. The risk is highest in the **first two weeks after the flip**. *Grower practice*; the biology behind it is *Supported*.

## Why

Cannabis flowers in response to long, uninterrupted nights. Inflorescence development needed at least three consecutive short days in one study, and moving plants back to long days raised [gibberellin](https://genetixbay.com/knowledge/glossary/gibberellin.md) and [auxin](https://genetixbay.com/knowledge/glossary/auxin.md) at the [shoot tip](https://genetixbay.com/knowledge/glossary/apical-meristem.md) and pulled the forming inflorescence apart [1]. That's [re-veg](https://genetixbay.com/knowledge/glossary/re-veg.md) in slow motion.

One long day doesn't undo weeks of induction in an established flower, but it does stall it. Several days can start reversion. Even dim stray light delays flowering in sensitive cultivars [2][3], so a full day of light is a bigger signal.

**House position:** [light stress](https://genetixbay.com/knowledge/glossary/light-stress.md) reveals [herm-prone](https://genetixbay.com/knowledge/glossary/herm-prone.md) genetics; it doesn't make a stable plant [hermaphrodite](https://genetixbay.com/knowledge/glossary/hermaphrodite.md). A plant that throws [pollen sacs](https://genetixbay.com/knowledge/glossary/anther.md) after one bad night is the problem, not the timer.

Extra "reset" darkness is a cousin of the darkness-before-harvest *Myth*: no evidence, and a day of lost [photosynthesis](https://genetixbay.com/knowledge/glossary/photosynthesis.md).

## The numbers

| What happened | Likely effect | Action |
|---|---|---|
| One 24 h light period, mid or late flower | delay of about a day or two | resume schedule |
| One 24 h light period, first 2 weeks after the flip | induction stalls, more [stretch](https://genetixbay.com/knowledge/glossary/stretch.md) | resume; count from the restart |
| Several days of light | possible re-veg, single-finger leaves | resume; expect 1–2 weeks added |
| Watch window for pollen sacs | 2–3 weeks | inspect every few days |

## What to do

1. Reset or replace the timer and return to normal at the next lights-off.
2. Note the date in your journal and adjust your harvest estimate.
3. Inspect plants every few days for 2–3 weeks for pollen sacs.
4. Remove isolated sacs; cull a plant that keeps producing them, and never breed from it.
5. Fit a timer you trust, and check it after every power cut.

## Common mistakes

- Giving 24–48 h of darkness "to reset".
- Panic-harvesting early.
- Excusing [late-flower bananas](https://genetixbay.com/knowledge/glossary/nanners.md) as normal.
- Not checking the timer after a power cut.

## Related

- Full article: [Photoperiod and light leaks](https://genetixbay.com/knowledge/environment/photoperiod-light-leaks.md)
- [Can I open my tent during the dark period?](https://genetixbay.com/knowledge/answers/opening-tent-during-dark-period.md)
- [Why is my plant growing single-finger leaves?](https://genetixbay.com/knowledge/answers/single-finger-leaves-reveg.md)
- [Hermaphrodites](https://genetixbay.com/knowledge/diagnosis/hermaphrodites.md)
- Terms: [re-veg](https://genetixbay.com/knowledge/glossary/re-veg.md) · [nanners](https://genetixbay.com/knowledge/glossary/nanners.md)

## Follow-up questions

**Should I give extra darkness after the lights stayed on?** No, you don't need to give extra darkness after a light schedule failure in flower. Return to your normal 12/12 or 13/11 at the next scheduled lights-off. A long 'reset' dark period has no evidence behind it and cuts a day of photosynthesis. The plant measures each uninterrupted night, so normal long nights get flowering back on track.

**Will one night of light cause hermaphrodites?** One night of light can trigger pollen sacs on herm-prone plants, but in GenetixBay's breeder position the stress reveals unstable genetics rather than creating them: stable plants beside them stay female. Check the plants every few days for 2–3 weeks. Remove any pollen sacs, and cull and don't breed from a plant that herms. Late-flower bananas are not normal.

**What if my lights stayed on for several days in flower?** Several days of continuous light in flower can stall a photoperiod plant or start it reverting, with new single-finger leaves on the buds, because induction needs consecutive long nights. Return to 12/12 or 13/11 immediately; most plants resume flowering within about a week or two, adding that time to harvest. Plants early in flower are affected most.

## Sources

1. Alter H, Sade Y, Sood A, et al. 2024. Inflorescence development in female cannabis plants is mediated by photoperiod and gibberellin. Horticulture Research 11(11):uhae245. https://doi.org/10.1093/hr/uhae245
2. 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
3. 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