Indoor cannabis growers are always looking for ways to increase the health, growth, and output of crops. One method many growers use to accomplish this is to allow grow lights to stay on 24 hours a day. But is that the best option for your plants?

It’s not recommended to leave your grow light running for 24 hours straight. Twenty four hours of light can prevent a plant from effectively carrying out the respiration process. Long light exposure during the vegetative phase is more likely to encourage cannabis plants to grow into male plants.

If your plant is not responding to 12 or 18 hours of light, you may need a stronger grow light or you may need to feed your plant nutrients. Also consider whether you need to adjust your grow light hang height. Keep reading below for a more detailed explanation.

Advantages Of A 24 Hour Grow Light Cycle:

Faster Plant Growth

Running your grow light for 24 hours may force your plants to grow faster. More light equals more energy to fuel plant growth.

Stable Growing Environment

Continually running your grow light can also allow for more stable temperature and humidity levels. We know cannabis plants don’t like large swings in their environmental conditions, therefore this would be a positive.

Temporary 24 Hour Light Cycle

Some people choose to do this during the vegetative growth cycle to shorten the vegetative growth phase. When you’re ready for the plant to begin the flowering phase, you can transition to a more normal light cycle.

Disadvantages Of A 24 Hour Grow Light Cycle:

Long Light Exposure Is More Likely To Encourage Male Cannabis Plant Growth

Long time cannabis cultivator Jorge Cervantes recommends 14 hour light cycles during the vegetative growth phase. A shorter light cycle of 14 hours compared to 18 hours or longer will encourage the plants to grow as females.

This doesn’t mean you won’t be able to get female plants from 18 hours of light. It simply means plants are encouraged by their environment and more likely to grow as females under 14 hours of light compared to 18, 20, or 24 hours of light exposure during the vegetative phase.

If you’re growing unfeminized seeds and you want to give them an extra nudge to grow as females, consider this trick, and read my post about tips for growing more female cannabis plants.

Higher Growing Costs

Running a grow light for cannabis for 24 hours can be expensive. Not only will your electricity bill be higher, but you may need to feed more nutrients and water to sustain the increased rate of growth.

Less Time For Respiration

Although the plant respiration process can occur at any time, it won’t occur as much when the plant is exposed to light. 24 hours of light can cause your plants to become stressed and overworked, which can lead to stunted growth or other problems.

Respiration is when plants repair and strengthen cells, it’s an important part of the growth process that shouldn’t be neglected.

What Happens If You Accidentally Leave Grow Lights On Overnight?

One question I receive frequently is from people who accidentally left their grow lights running overnight (or many hours longer than planned) due to a timer breaking or forgetting to manually shut the grow light off.

During the Vegetative Growth Phase:

If your plants are in the vegetative growth phase they will be fine. Even though 24 hours of light is not recommended, it won’t kill your plants as long as they’re in the vegetative phase.

During The Flowering Growth Phase:

If your plants are in the flowering phase, 24 hours of light is a big problem. Cannabis plants are sensitive to photoperiods when flowering. In other words, more than 12 hours of light risks confusing your plant and causing it to re-enter the vegetative phase of growth.

One single day of constant light may not be enough to revert your plants back to the vegetative phase. But if the plants do revert, it’ll take weeks before it’s ready to re-enter the flowering phase.

Can You Give Autoflowers A 24 Hours Light Cycle?

Autoflowers, unlike their photoperiod cannabis plant cousins, only require one light schedule for the entire life of the plant. That means growers can leave the lights running 18 or 20 hours from seedling to harvest.

Can you give autoflowers 24 hours of light? As with other plants in the vegetative phase of growth, it’s possible but not recommended. Autoflowers need darkness to perform the respiration process.

As mentioned below, 24 hours of light will not fully prohibit respiration but it may reduce the process. 18 or 20 hours of light is the more preferable method.

Can You Give Seedlings 24 Hours Of Light?

Providing 24 hours of light to seedlings may force the seedlings to grow faster, but shouldn’t be done long term.

Cannabis seedlings grown under 24 hours of light take longer to show pre-flowers (and thus switch to flowering), than seedlings grown under 18 hours of light.

Seedlings grown under 16 hours of light showed pre-flowers even sooner than those grown under 18 hours of light. But I wouldn’t advise lighting for less than 16 hours during seedling/veg phases.

It’s better to give the seedlings 4 to 8 hours of darkness each day. If you can’t be around to switch off your grow light, consider a digital programmable grow light timer.

This timer is rated for grow lights up to 1700 watts of power total. It’ll turn off your lights for the desired amount of time each day so your seedlings can have darkness.

Why Do People Leave Grow Lights On 24 Hours A Day?

People naturally want the most yield from their plants, so they figure more light equals more yield. While they’re right about more light, they’re wrong about the method to obtain more light (leaving grow lights on for 24 hours).

This all comes back to the strength of your grow light and the ideal amount of light per day for your plants. I go into that in more detail below, but in short, if your grow light is too weak, you need to buy a stronger one, not keep your weak one running longer! Check out my review of the best LED grow lights if you’re in need of a stronger light. They’re fairly priced and offer good light intensity.

Can I Change My Grow Light Schedule From 24 To 18 hours?

Yes, and you can switch the plant light schedule from 24 to 18 hours during the vegetative growth phase without worrying about harm to your plants.

If you leave grow lights running 24 hours per day during the flowering phase, you’ll risk reverting your plants back into the vegetative growth phase.

Biological Processes Associated With Each Part Of The Light Cycle: Photosynthesis and Respiration

There are biochemical processes that occur in plants that directly relate to growth. Altering these can inhibit a plant’s natural growth pattern.

During the day, photosynthesis enables plants to absorb the sunlight and turn it into an unstable energy source. At night, the respiration process turns that unstable energy into carbohydrates which the plant stores for later use.

Photosynthesis cannot occur without light exposure. While respiration can occur at any time, darkness will trigger this reaction as this is nature’s way telling a plant to eat, rest, and store enough energy to tap during photosynthesis the next day.

When growers expose plants to light 24 hours a day, they prevent normal respiration from happening, which can lead to unhealthy plants.

Although it’s not guaranteed, repeated 24 hour grow light cycles may ultimately end up affecting your cannabis yield. Not to mention, your LED grow lights would enjoy a rest too, despite being efficient!

What’s the ideal amount of light exposure for plants?

The ideal amount of light exposure for fruit or vegetable bearing plants, including cannabis, is between 20 and 40 moles of light per day.

During an 18 hour vegetative phase light cycle, you can produce 20 moles of light by using an LED grow light that emits 308 micromoles of light for 18 hours straight.

To produce 20 moles of light during flowering phase, you’ll need a higher PPFD value of 462 micromoles from your grow light due to the shortened 12 hour lighting cycle.

If you want to maximize yield you’ll need to produce 40 moles of light per day for your plants.

To deliver 40 moles of light during vegetation, that’ll require using a grow light with a minimum PPFD value of 617 micromoles for 18 hours.

To deliver 40 moles of light during flowering phase, your grow light will need a 926 micromoles PPFD value to compensate for the shorter 12 hour light cycle.

Click here to read my article about sufficient PPFD values and grow lights that provide sufficient PPFD intensity to your plants.

Potential Problems with 24 hour grow light exposure:

The consensus among indoor gardening experts is that it’s critical you don’t leave the lights on constantly throughout the life cycle of most plants.

The lack of rest for plants can lead to weak root growth and fruiting ability. 24-hour grow lights can overwork a plant and diminish long-term health.

Can You Be Flexible With Cannabis Photoperiod Light Cycles?

People understand the general rule of thumb for growing photoperiod cannabis is 18 hours of light during the vegetative growth phase, and 6 hours of rest.

The rule of thumb for the flowering growth phase is 12 hours of light, 12 hours of rest.

Readers have asked me whether they can be flexible with the duration of light exposure they provide their plants. The answer is yes – but only to a certain extent.

The vegetative phase of growth is the most flexible, depending on the intensity of your light, you may be able to get away with 16 hour photoperiods. You can definitely get away with 20 hour photoperiods if you’re using a less intense light source.

If your plants are in the flowering phase, I wouldn’t recommend veering off the recommended 12 hour light cycle by much. Maybe 1 hour on either side (11 or 13 hour days). But there’s a small amount of flexibility there.

The amount of time you expose your plant to light doesn’t really matter. What ultimately matters is the total amount of light molecules your plant receives on a daily basis.

Going back to my article about Understanding DLI and PPFD, you’ll see that fruit, vegetable, and cannabis plant yields are affected by the amount of light molecules, not the duration of light exposure.

In summary, if you have a weak intensity grow light, you can consider extending the photoperiod as I discussed above, or consider buying an additional grow light. Check out my home page for the current best LED grow lights on the market!

At the end of my video here I discuss the topic a little further:

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