Can You Leave Grow Lights On 24 Hours A Day?

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If you are new to growing your own marijuana, you probably have a heck of a lot of questions that you want answered. Most people do have tons of questions when it comes to grow lights and lighting requirements for marijuana plants. Can you leave grow lights on 24 hours a day? It’s one of the main questions that people often ask us, and we will do our best to answer this right now.

To keep it short, Yes, you can technically leave grow lights on 24 hours per day, but this is only during the vegetative state of growth. Some people choose to only leave the lights on for 18 hours per day, even in the vegetative state of marijuana growth, which is for good reason.

It’s something we will explain below, but for the flowering stage of marijuana growth, no, grow lights cannot be left on for 24 hours a day.


Leaving Weed Grow Lights On For 24 Hours A Day

cannabis plant lighting

Now, you might have heard how many people out there leave their weed grow lights on for 24 hours a day, or 24/7, especially for those first weeks or few months of the vegetative growth stage. Yes, technically you can do this, and depending on the weed strain you are growing, it should be fine, more or less.

If you do leave the grow lights on 24/7 during this stage, your pot plants will absorb a heck of a lot of light and will be constantly engaged in photosynthesis.

This in turn will then lead to maximum growth, both in height and width, as well as the maximum growth of foliage or fan leaves. However, what you need to know here is that bigger and faster growth is not always better, but why is this?

Issues With Leaving Marijuana Grow Lights On 24/7

Ok, so there are some issues that may arise if you leave your pot grow lights on 24 hours per day. The issue is that most plants, yes pot plants too, do benefit from a period of darkness.

Just like humans and animals, plants need to rest as well. If a pot plant is constantly exposed to light during the vegetative state, it may become unhealthy and stressed. Simply put, although not 100% accurate per day, your plants need to sleep, more or less.

A pot plant needs time where it can rest, where it is not engaged in photosynthesis. Prolonged 24/7 exposure to light can cause stress, and it cause your weed plants to become more susceptible to stress damage, disease, and may also lower resistance to pests and illnesses.

Side note: if you need some help finding a good full spectrum light, we have reviewed our favorite ones here.

The Ideal Light Cycle IN The Veg State – 18/6

For this reason, plants needing rest, most pro marijuana growers would recommend providing weed plants in the vegetative state with 18 hours of light per day, and 6 hours of darkness, to let them rest and recover.

It’s kind of the same with humans. Have you ever been awake for 24 hours? Have you been awake for 48 or 72 hours? Yeah, it gets exhausting and in all reality it’s just not sustainable.

Sure, your weed plant might not grow as quickly or as tall with 18 hours of light per day instead of 24 hours of light, but it will also be much healthier.

If you are worried about your pot plant not growing as tall, simply leave it in the vegetative state for an extra week or two with an 18/6 hour light cycle to make up for the darkness periods.

Flowering Marijuana Plants & The Light Cycle

Alright, so, if you choose to leave your grow lights on for 24 hours per day, all the power to you. Most likely, things will turn out just fine, but you do have to be vigilant. You have to take increased care of your weed plant if you do this, and this goes for all growth aspects.

Your plant will be a bit weaker, or it might be anyway. However, what about the flowering state, when the buds start to form and get all nice and stinky?

No, for the flowering stage of marijuana growth, you absolutely cannot leave the grow light on 24 hours a day. In fact, if you leave the grow light on for 24 hours per day, the weed plant will remain in the vegetative state and won’t enter the flowering state, and thus will not grow flowers and buds. This is because all plants, including weed plants, have a natural mechanism.

They can tell when days get shorter outdoors or in other words, when the colder seasons are approaching, and therefore, when there is less sunlight per day, so they can achieve full flowers before it gets too cold out, the shift in light and darkness times cause the weed plants to begin flowering.

However, in an artificial indoor environment, if you provide your weed plants with 24 hours of light per day, there is nothing to signals your pot plants to start flowering, which is even true with 18 hours or light per day.

To begin and sustain marijuana flowering, they need 12 hours of light per day and 12 hours of dark.


Folks, it is possible to leave your grow lights on for 24 hours a day in the vegetative state of growth. Yes, your marijuana plants will grow fairly large and there will be an increased amount of foliage present, as well as stem height achieved.

Although, the problem is that most plants, including pot plants, do benefit from a light/dark cycle, or in other words, they are generally healthier and stronger when they receive some dark.

On that same note, during the flowering stage of growth, a weed plant needs 12 hours of light and 12 hours of dark in order to flower properly and produce big and potent buds, ones that will be ideal for smoking, making edibles with, and whatever else.


My passion for the sticky icky started nearly a decade ago, and it all began when I first laid my eyes on the beauty that is the marijuana plant.

I cover all aspects of growing from equipment recommendations to plant health/care tips to help both new and experienced growers.