Can You Leave Grow Lights on 24 Hours a Day? | Learn About the Ideal Light Cycle Range

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by Morgan

Last update:

August 29, 2023

How To Choose The Right Lighting For Your Marijuana Grow Room

Grow lights are essential for indoor plant growth, yield, and growth season extension. Leaving them on 24hrs a day makes a lot of sense.

Unfortunately, too much light can negatively impact your plants because it forces them to proliferate. So the answer to this question is no, it's not encouraged to have artificial lights on 24 hours a day.

Ideal light cycles should range from 12 or 18 hours a day, as extreme light exposure in different growth phases interrupts a plant's natural process of dormancy and growth.

Should your plants require more light, avoid damage by getting more fixtures or exchanging the existing ones for more powerful lights.

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24-Hour Grow Lights: Yield Killer Or Killer Yield?

Most indoor growers tend to gravitate towards longer hours of grow lights. Some even contemplate leaving grow lights 24/7 during the vegetative stage for constant photosynthesis.

24-hour grow lights results in maximum foliage growth from a technical standpoint, but it's highly dependent on your cannabis plant species and might cause plant stress. Plant stress due to extreme exposure might cause diseases, pest attacks, and death.

Constant exposure to growing light can slow down and prevent a plant from executing the respiration process. There are also possibilities of the cannabis plant growing into male plants due to this effect.

Pot plants absorb sufficient light, resulting in maximum width and height, leaf growth, and fan growth. Killer yield can, however, be produced and maximized with set lighting schedules per day.

What Happens During Plant Exposure?

On the one hand, the vegetative phase allows cannabis growers to leave LED grow lights 24 hours a day. On the other, a dark period is necessary to facilitate the flowering phase of fruit plants and vegetable bearing plants.

When growing plants indoors, it's necessary to reposition the dark vs. light exposure when the life cycle transitions. Being able to manage cannabis plants requires more than just light exposure.

marijuana plant under the light

Light-Dark Cycle Length Versus Plant Growth

Irrespective of the source, plants grow with at least 8/16 hours long light. Cannabis cultivation calls for darkness every six hours.

Most plants, if not all, require energy for glucose production. Glucose is plant food, and lighting conditions and concentrations vary from one plant species to another.

How to use Grow Light for Indoor Plants

Unlike humans, plants utilize light differently. Photosynthesis employs light to produce the energy required for growth.

Grow lights are fitted to emit more light in the spectrum that plants need for growth. For LED grow lights to have any effect, the positioning must be close and without obstructions.

It’s advisable, just like in nature, to fit your grow light above your plant for upward growth.

Flowering Stage

Stinky and more potent flowers start forming during this phase. Lamps facilitate bud growth into more giant flowers for consumption. Large-scale indoor growth means more power consumption.

This stage is mainly focused on increasing plant height, and both light and darkness have to repeatedly alternate for 12 hours a day each.

Vegetative State

The vegetative phases require light for growth. Marijuana plants have to rest for six hours before the light exposure of 18hours. At this state, less light ensures growers achieve healthier growth for solid stems and leaflet development.

The Ideal Light Cycle in the Vegetative State – 18/6

Since marijuana plants require rest at this stage, growers advise for 18 hours of light and 6 hours of darkness. The growth cycle slows down, but in return, growers will produce plants that are healthier and less harmful.

Should Seedlings Get 24 Hours of Light?

The growth environment during the earlier stages of weed growth must be flawless. With constant sunlight, marijuana seeds will have trouble germinating.

A light-dark cycle ensures a rest period and enough light on seedlings for 16 straight hours. Darkness ensures that the sprouting plant receives normal respiration for efficient growth.

Types of Grow Lights

Indoor gardening of marijuana requires good light intensity, and there are various LED grow lights you can choose from.

The 24-hour light exposure is essential during the vegetative phase, but the rest of the growth process requires a light and dark cycle balance.

LED(Light-Emitting Diodes)

LED grow light consumes minimal light energy levels and generates high output. It produces wavelengths of the complete light spectrum to support photosynthesis and all growth phases.

However, one downside of LED grow light is that they can cause optical damage and are more expensive than regular light bulbs.

Regular Light Bulbs

Aside from being long-lasting, regular light bulbs are energy efficient and cheap, unlike LEDs. However, their spectral wavelengths are narrow and heat up quite quickly and can slow down weed plants that need to start flowering.

Compact Fluorescent Lamps

CFL is a type of fluorescent lamp initially designed as an alternative to halogen lamps. They come in two options: plugin and screw in.

CFLs can be fitted 6 to 7 inches away for maximum result. A downside of the CFL lamps is their lack of cooling systems.

High-Intensity Discharge Lights

HIDS is among the significant grow light in the market today. Tight budget? The HIDs are inexpensive compared to their counterparts in this field.

The light output of HIDs is, however, to some extent less powerful, and they are more susceptible to heat than LEDs. Mirror foils come in handy to prevent light burn by reflecting light to the foliage.

Metal Halide Lights

Metal Halide lights emit light in blue and violet, similar to the colors outside during spring. By mimicking the sun's color spectrum, these grow lights make growing indoors absolutely fun.

Metal Halide lights churn out(60-125) lumens of light per watt, depending on the bulb.

The Optimal Exposure Light for Plants

The ideal light exposure amount for vegetable-bearing plants (cannabis included) ranges between 20 to 40 moles on a given day.

Growers can produce 20 light moles in an 18-hour phase using LED grow lights with an umol value of 308 during the vegetative phase. Consequently, they can produce 40 moles with umol values of 617 in this phase.

During the flowering stage, higher PPFD is required with values of up to 462 umol needed to raise 20moles of light, and 40 moles will require 926 umol PPFD. To maximize yield, growers should harness 40 light moles in 12-hour cycles.

The Effect of Keeping A Plant Under Constant Light

When weed plants take in a lot of light, photosynthesis constantly occurs. Constant light has a somewhat positive impact, resulting in maximum foliage growth.

However, constant exposure during the vegetative stage might lead to plant stress. Plant stress exposes weed plants to diseases, making them susceptible to pests and death.

The Effect of Light on Respiration and Photosynthesis

Daytime: Sunlight absorption and conversion into an unstable energy source in a  process known as photosynthesis, and it is not possible without exposure to light.

Nighttime: Unstable energy harnessed from the sun is converted into carbohydrates for future use through respiration. The onset of darkness triggers a plant to commence this reaction and store enough resources to tap back into the following day during photosynthesis.

Constant exposure to light prevents respiration resulting in unhealthy cannabis plants and ultimately low yield.

marijuana plant growing

Flowering Marijuana Plants & the Light Cycle

Each plant has a light cycle. For instance, plants with excess flowers may restrict plant growth; hence exposure to too much sunlight and the LED grow lights must be kept on for more extended periods, hindering the fruiting cycle.

Twenty-four hours a day of light hastens growth, but subsequent exposure to darkness ensures that the flowering phase commences.

Horticulturists worldwide have perfected calculating their lighting needs to maximize the yields of their marijuana plants.

Should Autoflowers Have 24 Hours of Light?

Unlike their short-day cousins, the cannabis plant, auto flowers require a single light cycle for the plants' entire life. Also known as day-neutral, autoflowering cannabis strains automatically switch from the vegetative to the flowering stage.

Growers can be lights on for 18 /20 light hours from seedling phase to harvest phase.

Should you run 24 hours of light for an autoflower seeds? It's possible but not recommended due to the respiration limiting factors.

What Happens if I Leave Grow Lights on Accidentally for 24 Hours?

Plants in the vegetative phase are rarely affected by this phenomenon; however, plants' health in the flowering stage can be heavily affected.

Anything north of 12hours of light can affect the biochemical processes of growing cannabis and result in re-entry into the vegetative phase.

It might take weeks before re-flowering occurs again. Either forgetting to shut off the artificial light or timer breakdown causes this problem.

marijuana growing indoor

The Best Lighting Schedules for Cannabis Plant Cycles

You can switch your lights on up to 12 hours a day during flowering. Longer timeframes can cause reversal to vegging, and your weed plants will prove challenging to flower again. This light exposure method, however, does not apply to auto flowers.

During the vegging phase, indoor marijuana growth gains more flexibility. There is a possibility of leaving the led grow lights on overnight with some minimal dark hours.

Commonly the schedule used by most growers is 18/6 light vs. dark, respectively. We recommend using the 20 hours lights on and 4 hours off schedule.

Alternatively, you can use the 6/2 schedule, having 6 hours for light and 2 for darkness, three times a day.


For weed plants to effectively flower and produce healthier buds that are powerful and ideal for different uses, 12hours of light exposure and 12 hours of dark exposure are recommended.

Scheduled darkness can prove beneficial in the flowering state to prevent unhealthy plants, while forcing plants to grow faster by constant lighting might cause plant stress.

Should you experience stunted growth, replace your older lights with more efficient and powerful ones for more light.

🌿  Join my "growing" community 
of 5000+ happy
cannabis farmers! πŸ‘©β€πŸŒΎ

Receive my weekly articles in marijuana growing,
from seeds to smoke! 
β€” Morgan, 420GreenThumb 🌱

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About the author 


I discovered the benefits of Cannabis at a young age in 2002, with years of trial and error, my knowledge grew just like my plants. As my love for cannabis unfolded I began to teach and learn, trying to gain as much information and practical growing experience.

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