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Leaving Grow Lights on 24 Hours: Is it Recommended?

Indoor growers are always looking for ways to increase the health, growth, and output of crops. One method many have heard to accomplish this is to allow grow lights to stay on 24 hours a day.

It is 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.

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.

What If I Accidentally Leave My Grow Light On For 24 Hours?

One question I receive frequently is from people who accidentally left their grow lights running for 24 hours (or many hours longer than planned) due to a timer breaking or forgetting to manually shut the grow light off.

If your plants are in the vegetative 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.

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. If this occurs, your plant will take weeks before it’s ready to re-enter the flowering phase.

Why Do People Leave Grow Lights Running 24 Hours In The First Place?

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 their grow lights running 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 COB LED grow lights if you’re in need of a stronger light. They’re fairly priced and offer good light intensity.

Can I switch my plant light schedule from 24 to 18 hours?

Yes you can, 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.

Why Plants Need Both Light and Darkness:

In nature, plants thrive when they take root in advantageous locations. Plants that need 8-12 hours of daily sun will perform poorly, or even die, when they take root in a shady location.

Growing plants indoors provides the benefit of controlling the elements a plant requires for optimal growth, namely food, water, and light. But why is exposure to light critical?

Photosynthesis/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!

Light exposure and plant cycles:

Every plant has a growth cycle, and this is where it’s essential to know what a grower’s goals are.

A grower who wishes to produce plants with an abundance of blooms will inhibit that plant by exposing it to light 24 hours a day. Too much light can interrupt a plant’s natural process of growth, flowering, fruit, and dormancy.

On the other hand, 24-hour lighting can force plants to grow quickly. These plants can then transition to a more normal light/dark exposure when a grower desires the flowering and fruiting cycle to begin.

The ability to manage the life cycles of a plant with grow lights can increase the speed between crop production of flowers or fruit.

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 vegetation phase light cycle, you can produce 20 moles of light by using an LED grow light with a PPFD value of 308 micromoles (umol) of light.

To produce 20 moles of light during flowering phase, you’ll need a higher PPFD value of 462 umol 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 umols for 18 hours.

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


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


Types of Indoor Grow Lights:

The advancements made in grow lights are enabling indoor garden enthusiasts to grow plants in even the darkest of locations.

Quality grow lights produce a variety of light-spectrum wavelengths with a controllable level of intensity. The bulbs inside the lights can be incandescent, LED, or fluorescent.

When your goal is to produce seedlings, root crops, or vegetables, you need high-spectrum bulbs. Low-spectrum bulbs are best for fruiting plants, marijuana, or flowers. A full-spectrum grow light is ideal for those who produce a variety of plants.

Each bulb type emits heat, which is another factor to consider when exposing plants to light 24 hours a day. Indoor spaces with poor ventilation may experience a dramatic rise in temperature that could harm the plants.

Potential Problems with 24 hour grow light exposure:

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.

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 Light Photoperiods?

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

How Much Light To Grow Weed, Vegetables or Fruit during Seedlings, Vegetative, Flowering phases

FAQ:

Should I leave my grow light on 24 hours?

No you shouldn’t leave your grow light running for 24 hours straight.

24 hours of light healthy for plants?

24 hours of light is not healthy for your plants. Plants need to rest too.

What if 18 hours of light isn’t enough for my plants?

If 18 hours of light isn’t enough for your plants and you want to run your grow light for 24 hours, you need to buy a stronger grow light.
You can run the stronger grow light for fewer hours, allowing your plants time to rest. Check out my home page for the best grow lights in every price range.

How much light does my fruit, vegetable, or cannabis plant need?

Fruit or vegetable bearing plants, including cannabis, typically need between 20-40 moles of light per day. That translates to a grow light that emits between 460 and 925 uMols of light per second. Read my article here for more info.

References:

  1. Bennie, Jonathan, Thomas W. Davies, David Cruse, Kevin J. Gaston, and Nathan Swenson. “Ecological effects of artificial light at night on wild plants.” Journal of Ecology 104.3 (2016): 611-620.
  2. Chandra, Suman, Hemant Lata, Ikhlas Khan, and Mahmoud Elsohly. “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 (2009): 299-306.

  3. Backer Rachel, Schwinghamer Timothy, Rosenbaum Phillip, McCarty Vincent, Eichhorn Bilodeau Samuel, Lyu Dongmei, Ahmed Md Bulbul, Robinson George, Lefsrud Mark, Wilkins Olivia, Smith Donald L. (2019). “Plant Growth-Promoting Rhizobacteria for Cannabis Production: Yield, Cannabinoid Profile and Disease Resistance.” Frontiers in Plant Science 10. 10.3389/fpls.2019.00495

Nick
 

My name is Nick, and I'm a home grower who spends time writing about indoor growing tools and setups. My goal is for 420ExpertGuide to be a one stop shop for indoor growers to find the information they need.