Light cycles have always been a bit of a heated debate among indoor growers. Should I give my plants 18 hours of light and 6 hours off? Or should I leave them running 24 hours a day to get the most out of plants? What’s the deal with all this and why does any of it even matter?

These ideas are hotly debated, and there is a lot of confusion around it for most people. By the end of this article, I hope you will have at least a better idea about how plants see light, and why the answer to these questions depend on what plants you are trying to grow. 


What are the current theories on light cycles?

There are a few ideas being passed around on hydroponic forums as to what the best light cycle is for plants in the vegetative stage.

In general, there seems to be 2 main light:dark ratios. These are:

1. 18 hours on and 6 hours off (18:6)

2. Lights stay on 24 hours a day, every day (24:0)


The most common choice in the online community seems to be the 18:6 cycles.


People using this cycle suggests the following benefits:

  1. This cycle better simulates the light cycles found in nature

  2. Allows the lights to cool off during the dark cycle

  3. Creates a dip in the grow room temperature to mimic natural environments better

  4. Save energy

  5. Gives plants their needed sleep


For those using the 24 hours a day light cycles, the suggested benefits includes:

  1. Maximizing plant growth by delivering 24/7 energy production

  2. Keeps stable grow room temperatures

  3. Omits the need for timers and keeps lights running longer


How Do Plants See Light?

Plants have specialized proteins within their leaves that pick up light waves. There are many different kinds depending on the plant. These proteins are designed to react to the presence of light, and let the plant know what spectrum is reaching the leaf surface, and for how long. This information is then used by the plant to determine where to focus its growth.


A Plant's Circadian Rhythm

Plants don't have central nervous systems, but they do have their own version of a circadian rhythm. They decide what time of year it is depending on the ambient light cycles.

For plants, the night cycles are actually very important to maximize growth. Having a night cycle at the same time every day is important for a plant to understand what time of year it is, and how it should spend its collected light energy.

During the vegetative stage, when light cycles are over 12 hours a day, they usually focus more on growth than anything else. They add more leaves, grow thicker,  longer stems, and send their roots growing deep underground. This is all maintained by long day lengths. Most plants can actually be kept in a permanent vegetative phase simply by keeping them in this simulated “summer” long day duration environments.

Interestingly, plants don’t always measure the length of days. Many plants actually measure the length of night instead. It is this lack of light that triggers the plant to decide if it is time to begin preparing for winter or not.

Just as plants can be kept in the vegetative phase using light cycles. Plants can be stimulated to enter the flowering phases as well. This usually means spending its energy on thicker roots, flower production, and eventually fruit and seeds. This is important for indoor growers to consider, especially with crops where the flowers or fruits are the desired product.

The general agreement for most growers to initiate flowering is to change the light cycles to a 12 hour on, 12 hours off schedule. This tells the plants that winter is on its way, and causes them to prepare for this change in seasons by sending out its flowers.


There Are Exceptions to This

It should be noted here that there are indeed some plants that work off a different mechanism and will not be affected by day lengths. This is common with many tropical growing plants in fact.


Putting All Of This Together

Now that we have discussed the basics of how plants see light and what it means for growth, we can investigate what light ratios make the most sense for your own grow room. 

As we have mentioned, most plants actually measure the duration of night, rather than the duration of day. This means that a night cycle is at least to some extent needed by the plants to get its circadian rhythm in check. Choosing to go with a 24 hour a day light cycle may hurt the plant by making it impossible to develop; this circadian rhythm effectively. For smaller plants such as seedlings and clones, this does not seem to be a problem but can cause premature or weak flowering for fully developed plants when it comes time to flower.

The other cycle we mentioned, 18:6 more closely mimics the natural cycles of a more temperate climate in the summer. This light cycle allows the plants to develop a circadian rhythm with the 6 hours of daily night given to the plants. It also allows for a maximized amount of daylight that simply cannot be given in outdoor grows. This could perhaps be the best of both worlds. This cycle appears to have the most backing in the global online community and is used by professional growers worldwide as well.

Let us know your thoughts on light cycles and if there is anything we have missed. As mentioned earlier this is an area of hydroponics that is hotly debated, and there is likely no single right answer.