The duration of light a cannabis plant receives is the most important environmental factor determining its health, energy production, and life cycle progression. Light provides the energy for photosynthesis, converting carbon dioxide and water into the sugars required for growth and development. The specific number of hours of light needed changes dramatically depending on whether the plant is in its primary growth phase or its reproductive phase. Plants must receive a precise light schedule to ensure they transition into the stage that produces the desired compounds.
Understanding Photoperiodism in Cannabis
The cannabis plant, like many other flowering species, uses a biological mechanism called photoperiodism to determine when to shift from structural growth to flower production. This process is triggered not by the hours of light, but rather by the length of uninterrupted darkness the plant experiences daily. The plant measures the continuous night period, mimicking the shortening days of late summer and autumn in its natural outdoor habitat.
This time-keeping is managed by photoreceptor pigments. During darkness, these pigments slowly reset the plant’s internal clock. If the dark period is long enough, the plant releases flowering hormones, signaling the start of the reproductive phase.
This sensitivity defines photoperiodic strains, which require manual manipulation of the light cycle in controlled environments. Autoflowering strains do not rely on this mechanism; instead, they transition to the flowering phase based solely on their age and internal genetic programming. Autoflowering varieties contain genetics from Cannabis ruderalis and will begin to flower automatically, regardless of the light schedule they are given.
Light Cycles for Vegetative Growth
The vegetative phase focuses on maximizing leaf, stem, and root growth to build a strong foundation for future flower sites. During this stage, the plant requires a long period of light to maximize photosynthesis and energy storage. Growers typically provide between 18 and 24 hours of light daily to achieve optimal structural development.
The most common light schedule is 18 hours of light followed by 6 hours of darkness (18/6). While some growers use a continuous 24 hours of light (24/0) to accelerate growth, the 18/6 cycle is often preferred. This brief dark period allows the plant to efficiently process energy and nutrients, which can lead to healthier, more resilient growth.
Other variations, such as 20 hours of light and 4 hours of darkness (20/4), are also utilized, especially for autoflowering plants. The objective is to prevent the continuous dark period from exceeding the threshold that signals the start of the reproductive stage. As long as the plant receives more than 13 hours of light daily, it remains in the vegetative state.
Light Cycles for Inducing Flowering
To trigger the reproductive stage in photoperiodic cannabis, growers strictly enforce a light cycle of 12 hours of light followed by 12 hours of uninterrupted darkness (12/12). The 12-hour period of continuous darkness is the critical signal that initiates the hormonal changes necessary for flower production.
The darkness must be absolute and without any disruption for the entire 12-hour period. Even a brief flash of light, such as from an exterior light source or a cell phone screen, can interrupt the reset process. This interruption can confuse the plant, causing it to revert back to the vegetative state or induce stress that results in hermaphroditism (the development of male flowers on a female plant).
This strict 12/12 cycle mimics the natural shortening of the days near the autumnal equinox. The plant’s energy is then redirected away from structural expansion to focus entirely on developing flowers, terpenes, and cannabinoids. Maintaining this precise balance is necessary for a successful harvest.
The Difference Between Sunlight and Artificial Light
Natural sunlight is the gold standard for cannabis growth, providing a naturally balanced full spectrum of light and high intensity that artificial lights struggle to match. The sun’s spectrum includes all necessary wavelengths, from blue light that promotes vegetative growth to red light that encourages flowering, as well as UV and infrared light. However, sunlight is inconsistent, being subject to seasonal changes and weather conditions, which makes precise control impossible.
Conversely, artificial grow lights, particularly modern LED systems, offer complete control over the light environment. Indoor growers can adjust the duration, intensity, and specific color spectrum to match the plant’s needs at each stage. For instance, lights can be optimized to emit more blue light during the vegetative phase and shift to a higher concentration of red light to boost flower development.
For indoor cultivation, the concept of “hours of sunlight” is translated into the Daily Light Integral (DLI). DLI is a scientific metric representing the total amount of photosynthetically active radiation (PAR) light delivered to the plant canopy over a 24-hour period. Using high-intensity artificial lighting, growers achieve the necessary DLI for robust growth while maintaining control over the precise light duration required to trigger flowering.