Should You Be Leaving Grow Lights On 24 Hours A Day?

Providing sufficient light to your plants inside a grow space can be tricky, but using Grow lights enables your plants to thrive indoors and extend your growing season.

Plant grow lights are a great tool to make your growing process more efficient, but they can also be harmful to the plant if don’t follow the proper lighting schedule.

Now you might be wondering – how long should you keep grow light on or if it is recommended to leave grow lights on 24 hours?

The answer to the question is no, it is not recommended to leave your grow light 24 hours a day, because this will force plants to grow quickly. It is ideal to keep a grow light on for about 12 to 18 hours.

Too much light in various growth phases can interrupt a weed plant’s natural process of growth and dormancy. Worse, light exposure for a whole day can damage and kill your plants.

In this article, I’ll explain the consequences of leaving lights on 24/7 and the recommended hours to keep lights on depending on several factors.

This includes the plant type, temperature, type of light, grow size space, and plant’s growth stage. Once you learn the proper hours for light exposure, you can increase plant yield and profit.

Does More Light Mean More Yield?

Growers think that the longer plants get exposed to lights, the bigger their yield will be. Some even leave grow lights on 24/7 during the first weeks of the vegetative stage.

While they are correct about needing more light, the way of gaining light is incorrect. You have to consider some factors including the weed strain, type of light, and growth stage.

It is possible to leave grow lights on for 24 hours, but it is still way better to set lighting times per day. I will discuss more of this later on.

What Happens To A Plant That Is Kept Under Constant Light?

Leaving grow light on 24 hours a day on an indoor plant Pin

Like humans and animals, plants need a break from direct light, too. Imagine they need to sleep as well.

When cannabis absorbs a lot of light, it will constantly engage in photosynthesis. On the bright side, this will result in the maximum growth of foliage.

Yet, there are consequences as well. If your weeds are constantly exposed to light, especially during the vegetative stage, they may experience stress.

Once they get exposed to diseases, they also become susceptible to pests, and consequently, diseases and death.

Length Of The Light-Dark Cycle And Plant Growth

Plants thrive if they receive about 8 to 16 hours of light, regardless of whether it’s artificial light or natural light. Furthermore, growing cannabis needs at least 6 hours of darkness per day.

The same goes for any strain of weeds. They need water, carbon dioxide, and light to produce glucose, which serves as their food.le

When growing indoors, it is important to determine how to control light.

Keep in mind that various kinds of plants need different light exposure and concentration.

Understand Photosynthesis And Respiration In Plants

Grow lights mimic sunlight. Good thing that plants only need light itself.

Photosynthesis lets plants absorb light and convert it into an energy source. At night, when everything goes dark, respiration turns that energy into carbohydrates for later use.

This means that exposing your plants to light 24 hours a day will prevent respiration from occurring. No respiration can impact the health of plants.

What Happens When Plants Are Exposed To Light?

Full Spectrum grow light on an indoor plantPin

Plants have various growth cycles. As mentioned earlier, you may leave grow lights on for 24 hours when your plants are in the vegetative phase.

Meanwhile, plants grown for their flowers, fruits, and vegetables require a dark period to enter the flowering phase.

In other words, you must shift to the recommended exposures to light and darkness once plants transition to other stages.

Your capacity to manage the growth cycles using lights can increase a plant’s speed of production. As you can see, it is not always about turning the lights on all day to ensure growth.

What Is The Ideal Amount Of Light Exposure For Plants?

Fruit and vegetable-bearing plants, such as marijuana, must receive 20 to 40 moles of light daily.

For example, you can generate 20 moles of light for an 18-hour vegetation stage light cycle. This is the case if you are using LED grow lights with 308 micromoles of light.

If you want to give 20 moles of light during the flowering stage, your lights must have at least a PPFD value of 462 micromoles of light to handle the 12-hour lighting cycle.

Factors To Consider: How Long Should I Leave Grow Lights On?

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Here are important factors to remember when planning how long to leave grow lights on:

  • Type of Plant
  • Growth Phase
  • Type of Light
  • Temperature and Humidity
  • Size of Grow Space

Once you have established the connection among these factors, you should be able to get an idea about how long to keep the lights on plants.

Type Of Plants

It is better if you are familiar with the kind of plant. There are three types: low-light, medium-light, and high-light plants. A seedbox must show the plant type.

Low-light plants should get about 4 hours of artificial light. On the other hand, medium-light plants require about six hours of light exposure. Meanwhile, high-light plants need about 14 to 16 hours of light.

Growth Phases

Light and dark cycles depend on the growing stage. Each phase requires a specific duration of light exposure to successfully complete the growing state.

Cloning/Seedling State: Should Seedlings Get 24 Hours Of Light?

No, that is a terrible idea. This is one of the most critical phases of plant growth, so growing conditions should be ideal. Seeds would find it hard to germinate when exposed to constant light. Thus, the advisable light-dark cycle for seedlings is 16 hours on and then 8 hours off.

While it is true that seedlings need lots of light to grow, they also need darkness to survive. Since seeds are still ‘baby’ plants, they require a dark time to undergo respiration.

Vegetative State

The vegetative phase needs more light to sustain growth. But this also means the cannabis plants must rest for at least 6 hours.

Hence, the ideal light-dark cycle must be 18 hours on and 6 hours off. The plants will not grow taller than you expect them to be from 24-hour exposure. Yet, this cycle provides you a healthier growth.

As long as you follow the suggested light-dark cycles, the plant will be able to form lots of leaves and a solid stem.

Flowering State

For this phase, buds will begin to form nice and stinky. The lights should help produce potent and big buds that are suitable for smoking or making edibles.

A bigger area coverage doesn’t necessarily need bigger light wattage. Regardless of the type of light, here’s a general guideline of

The flowering stage won’t focus on growing taller, but it will make sure your plants get wider.

In this state, you can’t leave the lights on for 24 hours. Otherwise, the weed plants will stay in the vegetative stage. Thus, it’s best to follow the light-dark cycle of 12 hours on and 12 hours off.

Plants have mechanisms that let them know if there’s less light. The transition from lightness to darkness enables the plants to grow full flowers.

Type Of Grow Light

Since you are using artificial lights, cannabis will need longer hours of light exposure.

Although the vegetative phase benefits from 24-hour light exposure, the rest of the stages needs a light and dark cycle. The type of grow light also plays a key role in this matter.

LED Grow Lights

This type of light produces certain, yet complete light spectrum wavelengths. It supports photosynthesis, as well as all growth phases.

The best part is that although LED lights generate great power, they only consume low energy.

On the downside, LEDs can be damaging to the human eye. They are also pricier than regular lights.

Regular Light Bulbs

Three Different LED light BulbsPin

Like LED grow lights, regular light bulbs are energy-efficient and long-lasting. They are also inexpensive.

However, regular lights have narrow spectral wavelengths and get heated easily. Sometimes, it could be challenging for them to flower a growing space.

Compact Fluorescent Lights

You can place CFLs about six to seven inches away from your plants. The downside is that, unlike LED lights, CFLs are not fitted with cooling systems.

High-Intensity Discharge Lights

HIDs are the go-to lights for growers who are on a tight budget. Unfortunately, they are less powerful than LED lights.

In addition, they are more prone to heat. This is why I recommend using a mirror or foil underneath the weed plants. In this way, the light can reflect towards the foliage.

Metal Halide Lights

MH lights produce a large amount of light per watt. On the flip side, the bulbs emit a lot of heat, which may cook seedlings.

Temperature And Humidity

In indoor grow room with digital grow room humidifierPin

Lights impact temperature, and subsequently, humidity. It is ideal to keep the temperature inside a grow space between 72 to 82 degrees Fahrenheit. Meanwhile, relative humidity must be around 30% to 40%.

Size Of Grow Space

A bigger area coverage doesn’t necessarily need bigger light wattage. Regardless of the type of light, here’s a general guideline of how many watts you need for a growing area.

  • 1 x 1: 32 watts
  • 2 x 1: 64 watts
  • 2 x 2: 128 watts
  • 3 x 3: 288 watts
  • 3 x 4: 384 watts
  • 4 x 4: 512 watts
  • 4 x 5: 640 watts
  • 5 x 5: 800 watts
  • 5 x 6: 960 watts
  • 6 x 6: 1152 watts

How Far Should Grow Lights Be From The Plants?

A light fixture’s distance will depend on the type of bulb and growth stage. Depending on the bulb type, the distance can range from three to 60 inches away from the canopy. In effect, you prevent the lights from damaging the foliage.

Moreover, note that the higher the wattage, the farther the grow light should be away from your plants.  

Type of Grow Light

Distance From the plant

LED Grow Lights

12 to 24 inches

Regular Light Bulbs

12 to 19 inches

Compact Fluorescent Lights

3 to 12 inches

High-Intensity Discharge Lights

24 to 60 inches

Metal Halide Lights

24 to 30 inches

Signs & Symptoms Of Insufficient Light Exposure On Plants

Although I have provided a guideline, sometimes a bit of experimenting can change this. Also, watch out for obvious signs that your cannabis plants need light.

The most common sign is that the plant is growing visibly taller toward the light. It means the plant is reaching or begging for more exposure. In some cases, lack of light would result in stunted growth. Similarly, it could be that new leaves are not vivid in color.

How Not To Overexpose Your Plants To Light

We have seen how grow lights serve as a crucial tool in growing marijuana indoors. But overexposing also poses some risks.

Once you see the leaves start to curl, you will know that there’s too much heat. The branches may also appear to be leggy.

Think of it this way, humans have a routine of eating to gain energy and sleeping to take a rest.

The same goes for growing cannabis. They require a routine for getting light exposure and dark time for resting.

  • You can go old school and use an alarm to signal when to turn on and off the grow lights.
  • The cheapest way to remember the duration is by setting an alarm. But if you don’t mind spending some bucks, you can purchase a light timer. A timer will automatically turn the lights on and off according to your preferred schedule.  
  • Aside from that, put a thermometer to monitor the grow room’s temperature. You may also opt for a controller, which lets you manage the grow lights, temperature, and humidity altogether. 

Furthermore, check whether you have too many lights inside the grow space. It could also be that the grow lights are too close to the canopy.

Another way is to reduce light exposure or gradually turn off the lights. This will let the plants rest in a dark space to consume energy.

What If 18 Hours Of Light Is Not Working For My Plants?

Growing Plant under a LED grow light Pin

Don’t push the lights to run for 24 hours. Although plants can accept lots of light, the actual heat might be harmful. If this is the case, you should change to a stronger grow light. A stronger grow light can run for fewer hours.

For example, some regular light bulbs do not provide sufficient light exposure for your plants even after 18 hours. In this situation, you may want to consider changing to full-spectrum LED lights that mimic sunlight.

By doing so, there’s a guarantee that even if you are not leaving grow lights on for 18 hours, your plants have received the necessary red and blue lights to sustain growth.

I suggest using LED lights with wavelengths of 630 to 660 nanometers. This range enables the blue and red lights to boost plant growth.

Another way is to rotate your plants every week. If your grow lights tend to focus their intensity at the center, your foliage won’t get full coverage.

Plan Your Light Cycles

How long you will leave grow lights on would depend on factors such as the type of plant, growth stage, temperature, and grow space.

If you want to become a successful indoor grower, you have to invest in learning how to maximize equipment such as grow lights.

I have experienced how difficult it could be to set up a schedule for lights. But with the information from this post, you can do it faster and easier!

To sum it up, leaving grow lights on for 24 hours could impact a plant’s health. Like humans, plants need to rest as well.

Remember, only the vegetative state can tolerate 24 hours of light exposure. But all phases have recommended on-off cycles to let weed plants undergo photosynthesis and respiration.

Likewise, turning off grow lights for a few hours lessens operational costs, but without sacrificing plant yield.

Do LED Grow Lights Work?

Lighting is one of the most vital factors you must consider when growing plants indoors.

It impacts growth size, growth speed, and bud quality – leading to higher yields per wattage.

Light Emitting Diodes (LEDs) have been perceived as the newest technology to aid indoor growers. This lighting system has solved some of the drawbacks of High-Pressure Sodium (HPS) lights.

Basically, LED grow lights can produce great yields due to their ability to manipulate the spectrum and deliver light precisely, all while only using a reasonable amount of energy.

Did you know that you can get yields of about 0.5 to 1 g/watt by using quality LED grow lights?

This is because LED lights deploy UV-B light, which is responsible for producing strong, smelly, and sticky buds.

Despite their capacity to deliver a full light spectrum, LEDs last a long time and emit less heat.

Note that the effectivity of LED grow lights in marijuana growth would still depend on the wattage, distance, and temperature.

This is why we’ll discuss in the following subtopics how well led grow lights really work.

And how to use grow lights efficiently for indoor plants before you invest your hard-earned money.

What Are LED Grow Lights?

LED grow lights are electrical devices that produce a spectrum of energy-efficient light, which can encourage plant growth.

They can either be the main source or supplementary source of light to help weed plants reach their full potential.

LEDs emit a spectrum of light that is akin to the sun’s rays. In some models, they only have specific light spectrums that may drive different stages of cannabis growth.

The good thing is that LEDs can become the only source of light for weed plants. In some cases, they could act as a supplement.

LED grow lights make it possible for cannabis growers to have indoor grow spaces. Thus, you can produce marijuana all year round.

How Do LED Grow Lights Work? 

How an LED Grow Light WorksPin

LED grow lights pass light energy through semiconductor chips to radiate their light spectrum. There’s no need to use gas, lead, filaments, or mercury to produce light.

They have two types of semiconductor materials that can generate their own light spectrum.

The first semiconductor has negative charges or electrons. Meanwhile, the second semiconductor has positive charges called holes.

This setup makes LEDs versatile, durable, energy-efficient, and environmentally friendly.

How Efficient Are LED Grow Lights?

Using LED grow lights is a wise decision for indoor growers because they deliver the necessary spectrum, low heat, and durability, which are essential for maximum yield. Also, they emit more light per watt than HID or fluorescent lighting.

Unlike HID lights, LEDs can be utilized to emit a specific light spectrum. Hence, your cannabis gets the necessary color spectrums at different stages of growth.

There are particular wavelengths that are crucial for cannabis growth. They play a role in photosynthetically active radiation (PAR).

LED grow lights can create and focus PAR on marijuana to encourage marijuana growth. As a result, various wavelengths within PAR drive cannabis growth in different stages.

When the wavelengths touch the cannabis surface, there’s an intensity called Photosynthetic Photon Flux Density (PPFD).

Preferably, your LED grow light should have a PPFD value from 460 to 925 across your entire canopy.

For vegetative state, you can use white and blue LEDs. Meanwhile, the flowering stage would need red spectrums.

Factors To Consider When Using Led Grow Lights For Indoor Plants

LED grow lights emit wavelengths that encourage cannabis growth, thereby ensuring you get quality results.

Their ability to create an even light source ensures your whole canopy is receiving the necessary light for growth.

Since a LED lighting system provides a custom spectrum of light, they emit a more direct light, which is ideal for indoor cannabis growing spaces.

Size Of Your Grow Space

The size of your grow space also determines the number and size of LED grow lights you need.

For instance, a typical 3×3 grow space could require a 600-watt LED fixture.



LEDs produce various wavelengths that encourage the different stages of cannabis growth.

A full-spectrum LED grow light produces a range of wavelengths similar to the sun, which makes it useful for all sorts of plants at different stages. You could also adjust the light output at certain stages.

Heat Output

A LED grow light’s heat output directly impacts cannabis growth, as well as the light’s durability.

You should avoid a high heat output since LED grow lights are typically positioned in close proximity with weed plants.

Similarly, low heat output is highly recommendable to prevent damaging the leaves. It will also make the LEDs last longer because it will take more time to burn out.

Since LED grow lights can be hung, you’ll only have to analyze the distance between the canopy and the lights.

Additionally, LEDs have built-in cooling systems. Therefore, you’ll have no problems with overheating.

So, How Well Do LED Grow Lights Really Work?

Cannabis growers can choose from a range of light sources: HID, fluorescent, and LED. The important thing is that your light source will help deliver your desired quantity and quality of marijuana yield.

However, using LED is different from using HID or fluorescent lights. Likewise, it will depend on your growing environment, but you can use two 125W LEDs or a single unit that emits 250W.

The important thing to consider is whether your LED grow lights have 3W chipsets, which are advisable if you want best yields without worrying about burning your cannabis plants.

Ideally, your grow space’s LED panels should be 30 to 60 watts per square foot to achieve the best results.

Do note that this would depend on the number of plants you have. Don’t worry, we’ll have a complete discussion about that some other time.

Even if a LED lighting system is capable of delivering even coverage, you can further help by making sure that you have an even canopy. In this way, you minimize shading and obtain proper light exposure across your growing space.

Moreover, the distance between the marijuana canopy and LED grow lights varies from model to model. Nevertheless, they should be placed at least 12 to 18 inches from the top of your plants.

Generally, those with 1W chips should be positioned at least 12 inches away from your plants.

Meanwhile, those with 3W chips or bigger must be placed at least 18 inches away to avoid light burn.

A good indicator of whether your LED grow lights are too close is if you see your plants turning yellow.

But since LEDs produce less heat, you can place them much closer to the marijuana canopy. Consequently, you increase the intensity of light.

Lastly, to make sure that your LEDs are suitable for your growing conditions, you can start with growing temperatures of 83 degrees Fahrenheit to 85 degrees Fahrenheit.

This is because LED lights do not have infrared light, which means you have to make up for the gap by increasing the growth space’s temperature.

5 Advantages Of Growing Plants With Led Grow Lights

Energy Efficient

LED lights consume 60% less energy compared to HID and fluorescent lights, but are still capable of discharging the same amount of light.

Since there’s no need to burn gas, LED grow lights emit less heat, which is helpful in the temperature of your growing space.

In addition, LEDs are capable of producing quality yields all year round, making the investment worth the price.

Even Coverage

A lighting system normally creates hot spots, which is where the light is strongest on a canopy. As a result, some parts of the canopy see stretching or rapid growth while less light goes to the outer edges.

But with the help of a LED light, you can point an even light source across the entire canopy, avoiding dead zones and hot spots.

Healthier Plant Growth

Did you know that traditional grow lights produce more IR rays, UV rays, and more heat that will cause your weed plants to dry up and burn?

LEDs can still create these rays and heat but at a significantly reduced amount.


Traditional lights reduce their durability due to their need to burn gas, filaments, or other elements.

On the other hand, LED lamps have a low heat output, which dramatically increases their lifespan. As a result, they can normally last up to 100,000 hours.

Cost Effective

LED grow lights are actually pricier than traditional grow lights, but their efficiency and durability make them cheaper in the long run.

Time To Shift To LED Lights

Knowing your growing conditions is the first step. Afterward, learning the proper way to operate LED grow lights is the best way to earn your desired yields.

Not only do LED lights work effectively on growing marijuana, but they also work efficiently on your tasks because you don’t have to worry about ballasts, ducting, fans, overheating problems, and big electricity bills.

It is important that you choose full-spectrum LED grow lights that are capable of adjusting the wavelength to match your weed plant’s needs in each growth stage.

Remember, LED grow lights will ensure there’s no need to sacrifice durability and efficiency in order to achieve quality produce.

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