24/0 Light Schedule: Pros and Cons
Continuous lighting for cannabis veg: what works, what costs, and what the science shows.
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24/0 is not a magic yield multiplier. You pay 25% more electricity than 18/6 and almost always get similar or worse results. It works for heat in cold rooms and gives seedlings steady light, but most growers see no yield advantage. The science on photoperiod extension is clear: beyond ~18 hours daily light, cannabis doesn't grow or produce significantly more. Marketing claims otherwise.
What 24/0 Light Schedule Actually Is
A 24/0 light schedule means grow lights stay on continuously, 24 hours per day, during the vegetative stage. It is used as an alternative to the more common 20/4 Vegetative Light Schedule or 18/6 (18 hours on, 6 hours off) schedules.
Cannabis is a photoperiod-sensitive plant: flowering in photoperiod varieties is triggered by extended darkness (short days), not by accumulated light hours. So a photoperiod strain in vegetative growth will not flower under continuous light — it will stay vegetative until you reduce day length to 12/12 to trigger bloom. Autoflowers, which flower by age rather than photoperiod, will proceed to flower on their own schedule regardless of light hours, including under 24/0 Strong evidence[1].
24/0 is sometimes confused with light stress. Cannabis, like most C3 plants, can photosynthesize continuously as long as CO₂, water, temperature, and nutrients are supplied Strong evidence[2].
Why Growers Use 24/0 (and What Evidence Supports It)
Common claims for 24/0:
- Faster vegetative growth and larger plants
- Higher final yields
- Warmer grow environment (no cool-down period)
- Simpler operation (no timer needed)
- Better for seedlings and cuttings
What the evidence actually supports:
Warmer environment: True. Continuous lighting keeps the tent warmer, which can be useful in cold basements or winter grows for maintaining stable root-zone temperature. Strong evidence
Simpler operation: True in principle, but a $10 timer removes any real advantage.
Better for seedlings and cuttings: Many propagators run 24/0 or 18/6 on low-intensity light (T5, low-wattage LED) because young plants benefit from steady, gentle illumination and warmth. Anecdote No controlled study comparing propagation success directly between schedules was found in the literature.
Faster growth and higher yields: Weak evidence. Controlled horticultural research on cannabis and other C3 crops shows that once daily light integral (DLI) is adequate, extending the photoperiod beyond approximately 18 hours produces diminishing or no gains in dry biomass, and can cause problems in some species Strong evidence[3][4]. A controlled study of cannabis under varied light intensity found no significant yield advantage from photoperiod extension; yield and potency responded to total photosynthetic photon flux, not day length Strong evidence[3].
The energy cost: Running 24/0 consumes approximately 25% more electricity per day than 18/6 while delivering the same or less total light energy (in daily light integral). Most growers who switch from 18/6 to 24/0 and back report equal or slightly better results on 18/6, likely because plants use the dark period for respiration, active root growth, and hormonal signaling Weak / limited[5].
When to Start and When to Skip 24/0
Situations where 24/0 may be worthwhile:
- Seedlings under low-intensity light (T5, low-wattage LED under 200 µmol/m²/s). Young plants tolerate continuous light and benefit from warmth.
- Rooting clones in a humidity dome. Steady light and no temperature swings support initial root development.
- Autoflowers. Some growers prefer 24/0, others run 20/4 Vegetative Light Schedule or 18/6. No clear winner exists in published comparisons Weak / limited[1].
- Cold environments. If your room naturally runs cold (below 18 °C / 64 °F), continuous heat from lighting can prevent thermal stress.
Situations where 24/0 is not recommended:
- Warm or hot environments. With tent temperatures already at or above 28 °C (82 °F), no dark period means no opportunity to cool. Heat stress accelerates.
- High electricity costs. If you pay premium rates (e.g., >$0.20 per kWh), the cost premium of 24/0 is hard to justify without yield gains.
- High-PPFD lighting. If you are running 600+ µmol/m²/s, the plant is already light-saturated during 18 hours. Adding 6 more hours of the same intensity wastes energy.
- Masking early stress. Plants under 24/0 sometimes hide early signs of nutrient imbalance, root problems, or pathogen pressure until damage is severe Anecdote.
How to Run a 24/0 Schedule, Step by Step
- Confirm your strain type. Photoperiod plants will remain in vegetative growth indefinitely under 24/0. Autoflowers will transition to flower on their genetic schedule regardless of light hours.
- Measure your environment under continuous light. Run lights 24/0 for 48 hours and log temperature, humidity, and soil moisture at various times. Aim for leaf-surface temperature of 22–28 °C (72–82 °F) and relative humidity of 50–70%. If temperatures climb above 30 °C (86 °F) during the second day, continuous light will cause heat stress; consider 18/6 or better ventilation instead.
- Set light intensity appropriately. For seedlings, target 100–300 µmol/m²/s photosynthetic photon flux density (PPFD). For established vegetative plants, 400–600 µmol/m²/s is sufficient; higher does not yield proportionally more growth Strong evidence[3][6]. Use a light meter to confirm PPFD at canopy level.
- Use a timer even if you think you won't need one. A 24/0 timer set to 24/0 provides a consistent fallback if a power outage occurs or if you later want to trial 18/6. Cost is negligible.
- Monitor daily for stress. Check for leaf curl (heat or light stress), unusually pale or dark new growth (possible nutrient imbalance from continuous uptake without dark-period respiration), or slow extension (root oxygen stress from no dark period). Adjust watering, feeding, or light height based on observations.
- Transition to 12/12 flowering (photoperiod varieties only). Switch directly from 24/0 to 12/12 Flowering Light Schedule. No gradual step-down is necessary, though some growers prefer a 36–48 hour dark period immediately before the flip to trigger a cleaner flowering response Anecdote.
- Track electricity use. Compare your utility bill for a month of 24/0 against a previous month of 18/6, accounting for fixture wattage and local electricity rates. This is the only honest way to know whether the extra hours are paying for themselves.
Common Mistakes
- Assuming 24/0 = guaranteed faster growth or higher yield. It does not. Yield is driven by total DLI, genetics, environmental stability, and nutrition — not by maximizing photoperiod. This is marketing folklore, not horticultural law Strong evidence[3][4].
- Ignoring heat buildup. No dark period means no thermal reset. Tent temperatures often creep upward 1–3 °C per day under continuous light if ventilation is not adequate. By week two, a 26 °C start becomes a 29–30 °C problem.
- Overfeeding or overwatering. Continuous photosynthesis can mean continuous nutrient and water uptake. Growers who do not adjust electrical conductivity (EC) downward or reduce feeding frequency often see leaf tip burn or salt buildup. Conversely, continuous transpiration means soil dries faster; watering schedules need adjustment.
- Running 24/0 through the entire grow cycle on photoperiod plants by accident. This won't cause flowering, but it will prevent it. Growers sometimes forget to switch to 12/12, then wonder why no buds appear after 8–10 weeks. Use a written flip-date reminder.
- Believing marketing claims. LED manufacturers sometimes state that their fixtures "enable 24/0 scheduling for maximum yields." This is marketing copy unsupported by peer-reviewed evidence. Maximum yield is governed by DLI and genetics, not by pushing the photoperiod to its limit.
- Skipping the timer. Power flickers, utility work, and human error happen. A timer ensures the schedule is consistent through interruptions.
Related Techniques and Comparisons
- 20/4 Vegetative Light Schedule — middle ground between 18/6 and 24/0; offers slightly longer light hours without the full cost.
- 12/12 Flowering Light Schedule — the standard transition for photoperiod plants from veg to bloom.
- Autoflowering Cannabis Cultivation — where the 24/0 vs. 18/6 debate is most active, since light schedule does not prevent autoflowers from flowering.
- Daily Light Integral (DLI) — the total photosynthetic photons delivered per square meter per day; the metric that actually predicts growth rate more reliably than photoperiod alone.
- Light Height by Stage — how to adjust fixture distance to maintain consistent PPFD as plants grow.
Sources
- Peer-reviewed Moher, M., Jones, M., & Zheng, Y. (2021). Photoperiodic response of in vitro Cannabis sativa plants. HortScience, 56(1), 108–113.
- Peer-reviewed Chandra, S., Lata, H., Khan, I. A., & ElSohly, M. A. (2008). Photosynthetic response of Cannabis sativa L. to variations in photosynthetic photon flux densities, temperature and CO₂ conditions. Physiology and Molecular Biology of Plants, 14(4), 299–306.
- Peer-reviewed Rodriguez-Morrison, V., Llewellyn, D., & Zheng, Y. (2021). Cannabis yield, potency, and leaf photosynthesis respond differently to increasing light levels in an indoor environment. Frontiers in Plant Science, 12, 646020.
- Peer-reviewed Velez-Ramirez, A. I., van Ieperen, W., Vreugdenhil, D., & Millenaar, F. F. (2011). Plants under continuous light. Trends in Plant Science, 16(6), 310–318.
- Peer-reviewed Sysoeva, M. I., Markovskaya, E. F., & Shibaeva, T. G. (2010). Plants under continuous light: a review. Plant Stress, 4(1), 5–17.
- Peer-reviewed Chandra, S., Lata, H., Khan, I. A., & ElSohly, M. A. (2015). Light dependence of photosynthesis and water vapor exchange characteristics in different high Δ⁹-THC yielding varieties of Cannabis sativa L. Journal of Applied Research on Medicinal and Aromatic Plants, 2(2), 39–47.
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Related
- 12/12 Flowering Light Schedule — The standard light schedule to trigger flowering in photoperiod cannabis indoors.
- 20/4 Vegetative Light Schedule — Running lights 20 hours on, 4 hours off during veg to prevent flowering and manage heat an...
- Autoflowering Cannabis Cultivation — Age-based flowering cannabis for faster harvests and simplified growing.
- Light Height by Stage — Vertical light distance control for stage-appropriate intensity.
- More Light Is Always Better — A half-truth from the LED transition that ignores photosynthetic saturation and economics.