Can I leave grow lights on for 24 hours?

Leaving Grow Lights On 24 Hours – Everything You Need To Know

Wondering should you leave grow lights on for 24 hours straight is one of the common questions when growing weed. Let’s explore this topic in depth and find the best solution.

With marijuana being allowed for more than just medical purposes in several countries, this industry is starting to blossom, literally. Many started growing weed at home, for many obvious reasons – with quality, lower costs, and safety being the most prominent ones.

As a conscious grower, you should ought to provide the best conditions for your plants to grow and bring the best yields. Lightning is one of the most important aspects of successful growth & plant development, so having it set in order is a must.

Let’s figure out should you leave grow lights on 24 hours and why!

The current state of the topic

Currently, there’s a lot of debate going on should you keep the light on 24 hours without stopping. The main reason behind this ongoing debate lies in the yields and plant development, alongside personal opinions and experiences of individuals.

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On the other hand, a lack of firm evidence is the biggest possible contradiction to all the opinions stated online, including message boards, forums, groups, and social media. It all ties together to the point that there’s no right answer.

After researching the topic for a while, I realized that people form their opinions based on their personal experiences, and base their assumptions on their previous experiences with growing.

Adding the internet to this equation as a factor, and a possibility for everyone and their dog to state their opinion makes finding the truth quite difficult.

For example, the cannabis growers in the States have separate opinions, and could be classified into two groups:

  • 24-hour lights on” group
  • Anti 24-hour lights on” group

The former group of weed growers claims that the photosynthesis is the core of growth & development, and it’s an absolute must for plants. These claims are coming from the basic definition of photosynthesis – which states that plants use the energy from the light (the sun) and transform it into usable energy – carbohydrates. By stimulating this process 24-7, growers ensure that the plant is growing to its highest potential and gain the most biomass possible.

We could agree that photosynthesis is a necessity for proper development and thriving, there’s no doubt about that.

On the other hand, the second group addresses the plant’s natural order of life & functioning. The 24-hour-lights-off is preached because the darkness allows the plant to breathe, consume the carbohydrates stored, and rest from intensive photosynthesis.

That’s where the microbes which are fed into the rhizosphere (root system + soil) get the proper resting period which makes them not “overwhelmed” by constant photosynthesis. This results in the healthier plant overall; less robust stems, bulbs, leaves, with more defense to diseases.

Furthermore, the end product that comes from a plant grown in cycles which involve darkness (18/6, 12/12, 14/10, etc) is of better quality, especially considering cannabinoid & terpene levels (both indicate the quality of the plant and the product – the more the better).

Digging deeper

The above-mentioned situation and divide of thought aren’t really precise & definite, because growing a plant involves many more factors & conditions, which could (or could not) be manipulated.

The plant itself is the first variable in the equation; many species react to light differently and require a different amount of it throughout the day/week/lifetime.

Since we’re focused on growing weed and best practices in this branch of the science (no pun intended), we’ll disregard the other types of plants.

By definition, the plant of cannabis classifies into photoperiod plants, which means it reacts to both light and darkness. There are also auto-flowering cannabis plants, which will be explained later.

Simplified to the core (there are huge studies about this field), this means that the cannabis grows under while being lit, and blooms during the darkness. When considering what is required for best yields and best end product, the combination of light and darkness in growing appears as the best one.

But, the situation isn’t that simple to come to a conclusion that easily. There are more factors included in the process of growing & proper lightning.

Deeper into the rabbit hole we go…

Acknowledging the above-mentioned, we have to address the plant next. Throughout the growth & development, the plant of cannabis goes through several phases, each having its distinctive requirements and best practices.

  • Vegetative stage
  • Flowering stage


Also, both stages have significantly different practices when it comes to the location of the plant: indoors or outdoors (more on that later).

The vegetative stage starts with planting the seed and starting with the initial sprout. During this process, the plant is developing stems, leaves, and overall size. A photoperiod cannabis plant can stay in the vegetative stage almost indefinitely, as long as it receives less than 11-12 hours of darkness per a 24-hour cycle.

This means that you can keep your plant (if you have the right type) in this growth & development state for as long as you’d like, and eventually reach an impressive size.

The flowering stage starts after the plant starts receiving 12 or more hours of darkness. During this process, the growth is not as quick as in the veg stage, and the plant starts developing buds. A properly educated and careful-enough grower can put this fact into good use, and maximize growth and produce, which is often the case in growing cannabis for commercial purposes.

Where do you grow?

One of the biggest differences in growing cannabis comes from the environment – outdoors or indoors. Both environments can stimulate the plant in a great way, but both will require specific conditions & equipment and will yield results based on your engagement and investing.

Growing outdoors: Pros & Cons

The natural way to grow influences when you’re going to plant, water, and releases you from the hustle of proper lighting. When you’re growing outdoors, you just have to be careful when you’re going to start.

It all comes down to the geographic zone you’re in. You should plant during the spring season when the nights are getting shorter. The plant will grow slower and start flowering when the nights start to get longer, in the colder part of the year.

You might not get the exponential growth and plant/bud size, but the plants will surely be healthier compared to the ones grown indoors. (presuming you’ll handle the fertilizing, watering, and overall care about your plant)

By growing this way, your costs will be much lower, and you’ll end with high-quality cannabis with a high level of THC.


On the other hand, you’ll have fewer buds per plant, and smaller plants overall, which might be ok if you don’t want to commit too hard and invest a good chunk of money into equipment.

Growing indoors: Pros & Cons

If you grow indoors, you have full control over the plant’s environment, such as the airflow, the temperature, and the type of lightning. Most importantly, you decide how long you’ll treat the plant with lightning cycles.

Speaking of lightning, there’s a big difference in types of lightning used in different stages of plant development. Having a fancy HID, CFL, or LED light can bring you tremendous growth and prosperity, but a poor choice can also bring your plant down and wither it to a low-quality bush.

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Lights can be classified in the spectrum & provide the required illumination and stimulation for the plant in a specific stage. Here’s the image that explains what is best for a cannabis plant in different growth stages (Courtesy of RoyalQueenSeeds).

Image Source: RoyalQueenSeeds.com

As you can see, the UV lights are the best during the vegetative stage, while the infrared spectrum stimulates the flowering cannabis plant the best.

It’s important to keep in mind that watering during the flowering stage must be on par with the lightning because the stronger infrared spectrum produces a lot of heat and dries out the plant.

Important: Never leave the infrared light on your cannabis plant for more than 12 hours.

By growing indoors, you can stimulate your plants to reach exponential growth, and keep them in a healthy state by providing perfect light spectrum and the perfect amount of lightning in all stages. (presuming you’re handling other areas of proper plant care)


On the other hand, you’ll have significant costs for equipment and the dreaded electrical bill.

Conclusion: leaving the lights on 24 hours – yes or no?

Now we get to process the deeper knowledge & information:

When considering all the above-mentioned facts, the answer to the burning question – should I leave the grow lights on for 24 hours – manifests itself:

Short answer: It depends on you – what your goals are, and what you’re trying to grow/produce.

The longer answer is as follows:

The 24-hour periods are very good for the vegetative stage if you’re trying to maximize the growth, but you should be careful not to overdo it because the plant won’t have the proper growth process and eventually will become prone to illnesses and will yield lower-quality results.

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