Artificial lighting is a powerful tool for indoor growers, allowing them to provide the energy plants need to thrive regardless of external weather conditions. However, a common misunderstanding is that more light always equals more growth, which often leads to the question of whether grow lights should be left on all the time. The answer is generally a straightforward “Yes, you should turn them off.” Plants rely on a natural day-night cycle, and the dark period is not merely a rest; it is an active phase of their biological process. Utilizing an automated timer to create a consistent period of darkness is fundamental for successful indoor gardening.
Why Plants Need a Period of Darkness
The dark cycle is an active and necessary component of a plant’s metabolism, often described as an internal biological clock or circadian rhythm. During the light period, plants perform photosynthesis, converting light energy into sugars, primarily in the leaves. These sugars are temporarily stored as starch within the leaf cells as they are produced throughout the day.
Once the lights go out, the plant switches from energy production to energy management and distribution through a process called cellular respiration. The stored starches are broken down into simpler sugars, which are then transported out of the leaves to support growth in other areas, such as roots, stems, and new leaves. This sugar translocation is significantly more effective during the dark period and provides the building blocks for structural growth overnight. If this dark cycle is skipped, the continuous accumulation of starch in the leaves can impede the photosynthetic machinery, leading to reduced efficiency the next day.
Darkness also plays a regulatory role through the phytochrome system, which acts as a light-sensing switch for the plant. The red light-absorbing form of phytochrome, Pfr, slowly converts back to its inactive Pr form during long periods without light, a process called dark reversion. This conversion is how plants measure the length of the night, which triggers important developmental changes, particularly flowering. An uninterrupted dark period allows the phytochrome system to reset, ensuring the plant progresses through its life cycle as intended.
Standard Light Cycle Schedules
The duration of the light and dark periods must be adjusted to match the specific developmental stage of the plant, directly influencing its growth trajectory. The initial phase of growth for seedlings and young clones benefits from a very long light cycle to maximize energy intake for root and foliage establishment. Many growers utilize an 18-hour light and 6-hour dark schedule (18/6) during this vegetative phase. This extended light period encourages rapid biomass accumulation, resulting in a larger plant structure before the onset of flowering.
Once the plant has achieved the desired size, the lighting schedule is typically adjusted to induce the reproductive stage in photoperiod-dependent species. The most common cycle for triggering flowering or fruiting is a strict 12 hours of light followed by 12 hours of complete, uninterrupted darkness (12/12). This precise 12-hour dark period is the signal that tells the plant the season is changing, initiating the shift in hormonal production that leads to flower development. Any interruption of the dark period, even a brief flash of light, can interfere with the phytochrome reset and potentially prevent flowering.
For autoflowering varieties, which do not rely on the dark period trigger, a light cycle of 18/6 or even 20/4 is maintained throughout their entire lifespan. Since their flowering is determined by age, not light duration, the main goal is simply to maximize the total amount of light energy received each day. However, even these plants benefit from a short dark period to allow for necessary rest and sugar translocation, making the 18/6 schedule a common choice.
The Effects of Continuous Illumination
Leaving grow lights on for 24 hours a day (continuous illumination) can lead to detrimental consequences for most plant varieties. One immediate problem is photoinhibition, which occurs when the plant’s photosynthetic apparatus is oversaturated with light energy. This constant bombardment can damage the chloroplasts and reduce the overall efficiency of energy conversion, stressing the plant and slowing growth.
Continuous light prevents the necessary metabolic shift that happens during the dark period, disrupting the efficient movement of starches and sugars. Without this cycle of production and transport, sugars accumulate in the leaves, signaling that growth should be slowed down. This disruption can lead to stunted development, leaf discoloration, or nutrient lockout.
A lack of darkness also disrupts the photoperiodic response required for flowering. If a short-day plant is kept under continuous light, the phytochrome system never fully resets, and the plant fails to transition into its reproductive phase. Running lights 24 hours a day also significantly increases electricity consumption and the heat load in the grow space.