Can you Leave Grow Lights on 24/7?

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As a rule, the most important aspect of growing marijuana is increasing your yields. In fact, growers are always searching for various methods to improve production.

One of the most often used method for increasing yields is to keep your grow lights on 24/7. However, there are many downfalls to giving a plant to much light.

Actually, many specialists agree that 24 hours of light can keep a plant from efficiently completing the respiration process. Bottom line, if your plants absorb a lot of light, they will be in a constant state of photosynthesis.

In result, too much photosynthesis can lead to damage to the plant and degradation of important plant proteins.

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

One of the most often asked question from new growers is from those who accidentally leave their grow lights on for more than 24 hours. Leaving the lights on for too long is actually a common problem.

It’s often due to a malfunction in the timer, the timer breaking, or forgetting to shut the timer off manually.

If the plants are in the vegetative phase, generally it is not an issue if they are overexposed light. Nonetheless, 24 hours of light is really not good for production.

Can you Leave Grow Lights on 24/7?

However, overexposure to light will not kill your plants.

Some growers believe that seedlings need 24 hours of light. But in actuality, many experts recommend that you give seedlings 12 hours of light and 12 hours of dark.

On the other hand, some specialist feels that your seedlings need 16 hours of light, depending on the variety.

Should Autoflowers Have 24 Hours of Light?

Autoflowering is in reference to the life cycle of a certain cannabis strain and genetics. An auto-flowering seed automatically flowers without having to go through a certain timed light cycle.

Once the short vegetative growth cycle is finished, the buds begin to form. Generally, the slight vegetative growth cycle lasts only 2-4 weeks, depending on the strain and conditions.

Since auto-flowers only need one light cycle, growers can give the plant 18-20 hours of light. In fact, the strain can handle this much light from seedlings to harvest.

However, it is still not recommended to leave the lights on for 24 hours. Overexposure to light may lower respiration and production.

Why do People Leave Grow Lights on for 24 Hours ?

For most growers, their main objective is production and yield. It stands to reason that the more light a plant gets, the more it produces. With this in mind, some growers leave their lights on 24/7.

Although it is true that more light will produce more yields, it’s the type of light that’s needs to be monitored. For the most part, the strength of a grow light and the amount of light given to a plant will produce better yields.

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If your plants have stunted growth, it is usually best to upgrade the type of grow light you are using instead of increasing the amount of light hours. Generally, you need to but a stronger grow light that has good light intensity.

Some excellent grow lights include

Flowering Marijuana Plants & the Light Cycle

All plants have growth cycles. In addition, all plants require their own particular light cycle. Knowing how much light a plant needs for a certain objective is crucial to all growers.

For example, a grower that wants to produce plants with an overabundance of blooms will constrain that plant by overexposure to light, including leaving their grow lights on 24/7.

Too much light can interfere with growth, flowering, dormancy, and fruit. However, giving a plant 24 hour lighting can make it to grow much faster.

After the quick growth stage, the plants can have normal light and dark exposure so the flowering and fruit cycle starts.

To produce a profusion of blooms, the plant needs to go without darkness and be exposed to light 24 hours a day. However, too much light can disturb the natural process of its various plant cycles such as growth, flowering, fruit, and dormancy.

Growing cannabis indica, marijuana vegetation plants, cultivation cannabis, background green, marijuana leaves, hemp CBD, top view

The flowering state is when the buds are forming and they begin to get stinky. During this stage it is crucial that the plant get the proper amount of light.

In fact, the grow light should never be left on for 24 hours. This much light will keep the plant in a vegetative state and it will not enter the flowering stage even if it has started budding.

Actually, all budding and flowering will come to a complete stop. It is when the plant has less light that it starts to bud. In fact, when a plant has less light it causes it to flower.

Going from 24 hours of light to 18 will help your plants bloom. Likewise, some experts recommend that you give the plants only 12 hours of light instead of 18.

The Dark Side of Photosynthesis

During the day, photosynthesis helps plants absorb variable energy from sunlight. During the night, the unstable energy is converted into carbohydrates which are stored for reserves.

This process is called respiration. When respiration is altered it can obstruct a plants natural growth pattern.

Photosynthesis cannot happen without the exposure to light. However, respiration can happen at any time.

It is the dark that activates respiration. Respiration actually activates the plants eating and resting period.

White Russian automatic variety of marijuana flower with grey black white background

It then helps the plant store enough energy for the next bout of light or photosynthesis.

Quite the reverse, when growers give plants to much light it prevents normal respiration. Over time, the plant becomes quite unhealthy.

In fact, too much light can actually reduce yields.

Does More Light Mean More Yield?

For the most part, more light means bigger yields. However, having a balance of light and darkness during the right times is crucial.

The most effective method to increase yields is to maximize the amount of light intensity that your plants receive. Also keep in mind that when a plant receives more light it also need more nutrients.

Thankfully, marijuana is a hardy plant and can tolerate quite a bit of light without affecting its health. The key is to be insistent as possible when selecting the distance between the light and the canopy.

Always test the light at various heights and read the manufacturers instructions. A good rule of thumb is to start at 12 inches and carefully watch the plants over the next day.

If the leaves seem to wilt or burn, then the light is to close. Adjust the distance until you have the perfect exposure.

For optimum yields, the light needs to hang at the distance just before light burn happens.

Length of the Light-Dark Cycle and Plant Growth

There are basically four growth cycles of a cannabis plant. Each growth cycle requires a certain amount of light and dark for optimum plant growth and higher yields.

  • Germination – cannabis seeds do not need light to germinate. In fact, they require complete darkness for successful germination.
  • Seedling – once the seeds have sprouted, the seedling will need a lot of light. Seedlings need 18 to 24 hours of light, depending on preference and the strain.
  • Vegetative stage – Indoor growers typically give their plants either 18 hours of light and 6 hours of dark each day or 24 hours of light with no darkness.
  • Flowering stage – the flowering stage generally requires 10 to 12 hour light/dark cycle.

In addition, cannabis does not respond well to extreme temperatures. For the best growth, avoid colder temperatures. During the vegetative stage keep temperatures in your grow tent between 68°F to 86°F (20-30°C).

During the flowering stage, ideal temperatures should be between 68°F to 77°F (20-25°C). Likewise, perfect humidity levels should stay between 40% to 60% humidity.


Yes, you can leave grow lights on 24/7. Typically this is done during the vegetative state of growth.

Moreover your plants will grow rather big and have a lot of foliage plus stem height.

Nevertheless, most plant including cannabis will greatly benefit from a light and dark cycle. In fact, your plants will be a lot healthier and robust.

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Likewise, during the flowering stage, cannabis requires12 hours of light and 12 hours of dark to produce good buds. Quite frankly, the most potent and large buds come from plants that have a balanced amount of light and dark.

Overall, 24/7 light can be productive during the vegetative stage of growth, just be careful not to overdo it. If your plants receive too much light during this stage, they will be prone to disease and have a lower yield.