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Can You Leave Grow Lights On 24 Hours a Day? Fact Or Myth

The lighter, the better, right? Well, not quite. 

In some situations, prolonged exposure to light can be advantageous. But more often than not, it can cause more harm than good. So, should grow lights be left on 24 hours a day? Let’s find out!

Can You Leave Grow Lights On 24 Hours A Day?

The answer is No, you should not leave grow lights on 24 hours a day!

Imposing too much light on your plants can interrupt their natural growth process and stunt their development. Leaving grow lights on for 24 hours may seem like a good idea to trigger fast plant growth and flowering, but it often has the opposite effect. 

What Are Ideal Photoperiods For Plants?

Before we get into the nitty-gritty, we’ll go over photoperiods. 

For newbie gardeners – a photoperiod is the length of time a plant is exposed to light in a day. In general, more light leads to more flowering, while less light suppresses it. Each plant falls into one of the three lengths of photoperiods:

  • Short day: Less than 12 hours of light
  • Neutral day: Approximately 12 hours of light
  • Long day: Over 12 hours of light

These categories describe the right amount of light your plant needs to grow. But aside from these, there are many other factors impacting a plant’s development, which we’ll cover below. 

Does All Plants Need Grow Lights On 24 Hours?

Some plants can tolerate 24 hours of direct light. However, most will need both light and darkness. 

Regular sunlight

Think of it like a person’s need for a good night’s sleep. You can stay up and burn the midnight oil many nights in a row and get a lot more done than if you’d worked for 12 hours. But after the third all-nighter, the lack of sleep will take a hard toll on you. Well, your plants need their beauty sleep too. They have different requirements based on where they originate from and their own biological clocks that are regulated by the sun.  


Seedlings, conversely, can withstand 24 hours of light at their rapid growth stage. But that doesn’t mean they should. Once they mature, you should gradually wean them off of light and expose them to more darkness. 

Some good things to come out of leaving grow lights on for 24 hours include:

  • No need for grow light timers
  • Plants receive more light 
  • Faster growth for seedlings
  • Quicker flowering 

On the other hand, some negative aspects include:

  • No time to rest and respire
  • Additional electricity costs 
  • Frequent bulb changes
  • A need for cooling equipment
  • Added stress on the equipment

What Happens If You Leave Grow Lights On 24 Hours A Day?

What Happens If You Leave Grow Lights On 24 Hours A Day?

If you’re still not convinced about the harmful effects of leaving your grow lights on indefinitely, let’s talk about how it can impact your garden. 

Firstly, can you overdo grow lights? The answer is no.

Namely, some light bulbs give off more heat than plants can handle in the long term. When this light beams on a plant’s delicate body for extended periods of time, it can irreparably damage the leaves. The plant will begin showing signs of stress like wilting, wrinkling, drying out, and discoloration. Essentially, the plant will burn. A separate group of plants will get bleached and take on a paler, whitish color. So, it’s of utmost importance to choose the right type of grow light for your plant and give it the best fighting chance. 

Remember, don’t overdo it! Give your plants what they need depending on their life stage, not more, not less. That way, you’ll ensure steady growth and a healthy plant. 

How Many Hours Should A Grow Light Be On?

In short, most indoor plants, including high-quality realistic artificial greenery, need 12 to 16 hours of light daily. But the ideal number depends on a few things, including:

  • The type of plant 
  • The growth phase
  • The type of light
  • The size of grow space
  • Temperature and humidity 

So, to determine your plant’s photoperiod, you’ll need to do some digging (metaphorically). Namely, find out where your plant naturally grows in the wild. That’s why it’s best to set up a timer on the lights over your plants. This ensures they’re getting the right amount of light-to-dark ratio and keeps them healthy and thriving. 

Another thing worth mentioning is making sure you’re protecting your sleep schedule and not just focusing on the growth of your plants. If your indoor garden’s grow space isn’t a dedicated greenhouse, but a living space, a lot of grow light might interfere and reduce the quality of their sleep. So, before you buy plants with long-day requirements, sort out the technicalities of the grow space to ensure it doesn’t mess with your well-being. 


So, you shouldn’t leave grow lights on for 24 hours. Although some plants or seedlings may temporarily benefit from a surplus of grow light, all of them need time to rest, just like humans. To ensure your plant is getting enough light to fulfill its growing needs, do some thorough research before buying it. Find out where it grows, what weather conditions it thrives in, and how you can best emulate its environment.

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Mike Rastus-Josiah

Mike Rastus-Josiah

Mike Rastus-Josiah is the Master Gardener of Swipe Garden. He has over 30 years of experience in horticulture, plant physiology, and sustainability practices. Mike wants to continue sharing his expertise with inspiring others to explore the wonders of gardening.