Leaving Grow Lights on 24 Hours: Is it Recommended?

Indoor growers are always looking for ways to increase the health, growth, and output of crops. One method many have heard to accomplish this is to allow grow lights to stay on 24 hours a day.

While plants will grow faster under constant light, do experts recommend it for overall healthy plants?

The answer is yes and no. What a grower wishes to accomplish with his crop will determine the amount and intensity of light the plants should receive.

To understand the benefits and pitfalls of 24-hour lighting, a grower needs further information.

Why Plants Need Dark and Light

In nature, plants thrive when they take root in advantageous locations. Plants that need 8-12 hours of daily sun will perform poorly, or even die, when they take root in a shady location.

Growing plants indoors provides the benefit of controlling the elements a plant requires for optimal growth, namely food, water, and light. But why is exposure to light critical?


There are biochemical processes that occur in plants that directly relate to growth. Altering these can inhibit a plant’s natural growth pattern.

During the day, photosynthesis enables plants to absorb the sunlight and turn it into an unstable energy source. At night, the respiration process turns that unstable energy into carbohydrates which the plant stores for later use.

Photosynthesis cannot occur without light exposure. While respiration can occur at any time, darkness will trigger this reaction as this is nature’s way telling a plant to eat, rest, and store enough energy to tap during photosynthesis the next day.

When growers expose plants to light 24 hours a day, they prevent normal respiration from happening, which can lead to unhealthy plants. Although it’s not guaranteed, repeated 24 hour grow light cycles may ultimately end up affecting your cannabis yield. Not to mention, your LED grow lights would enjoy a rest too, despite being efficient!

Light exposure and plant cycles

Every plant has a growth cycle, and this is where it’s essential to know what a grower’s goals are.

A grower who wishes to produce plants with an abundance of blooms will inhibit that plant by exposing it to light 24 hours a day. Too much light can interrupt a plant’s natural process of growth, flowering, fruit, and dormancy.

On the other hand, 24-hour lighting can force plants to grow quickly. These plants can then transition to a more normal light/dark exposure when a grower desires the flowering and fruiting cycle to begin.

The ability to manage the life cycles of a plant with grow lights can increase the speed between crop production of flowers or fruit.

What’s the ideal amount of light exposure for plants?

The ideal amount of light exposure for fruit or vegetable bearing plants, including cannabis, is between 20 and 40 moles of light per day.

During an 18 hour vegetation phase light cycle, you can produce 20 moles of light by using an LED grow light with a PPFD value of 308 micromoles (umol) of light.

To produce 20 moles of light during flowering phase, you’ll need a higher PPFD value of 462 umol from your grow light due to the shortened 12 hour lighting cycle.

If you want to maximize yield you’ll need to produce 40 moles of light per day for your plants.

To deliver 40 moles of light during vegetation, that’ll require using a grow light with a minimum PPFD value of 617 umols for 18 hours.

To deliver 40 moles of light during flowering phase, your grow light will need a 926 umols PPFD value to compensate for the shorter 12 hour light cycle.


Click here to read our article about sufficient PPFD values and grow lights that provide sufficient PPFD intensity to your plants.


Types of Indoor Artificial Lights

The advancements made in grow lights are enabling indoor garden enthusiasts to grow plants in even the darkest of locations.

Quality grow lights produce a variety of light-spectrum wavelengths with a controllable level of intensity. The bulbs inside the lights can be incandescent, LED, or fluorescent.

When your goal is to produce seedlings, root crops, or vegetables, you need high-spectrum bulbs. Low-spectrum bulbs are best for fruiting plants, marijuana, or flowers. A full-spectrum grow light is ideal for those who produce a variety of plants.

Each bulb type emits heat, which is another factor to consider when exposing plants to light 24 hours a day. Indoor spaces with poor ventilation may experience a dramatic rise in temperature that could harm the plants.

Potential Problems with 24-Hour Light Exposure

The consensus amongst indoor gardening experts is that it’s critical you don’t leave the lights on constantly throughout the life cycle of most plants.

The lack of rest for plants can lead to weak root growth and fruiting ability. 24-hour grow lights can overwork a plant and diminish long-term health.

  • Nick
  • Updated May 20, 2019