Starting seeds indoors often requires using grow lights to supplement natural light. A seedling is a young plant that has recently germinated and transitioned to producing its own energy through photosynthesis. New growers frequently ask if leaving the lights on for a full 24 hours (a 24/0 schedule) will accelerate growth. While extra light seems beneficial, plant biology dictates that a dark period is necessary for a healthy growth cycle.
The Biological Necessity of Darkness
Plants, like most living organisms, operate on an internal timekeeping system known as a circadian rhythm, which regulates various biological functions over a 24-hour cycle. This rhythm relies on a consistent cycle of light and dark to function correctly and guide growth processes. During the light period, plants perform photosynthesis, converting light energy into sugars which serve as the building blocks for growth.
The dark period is not merely a time of rest but an active, metabolically important phase for the plant. Cellular respiration, the process of breaking down stored sugars to release usable energy (ATP), occurs constantly, but it is especially significant in the dark. This process uses the energy reserves created during the day to power essential functions like nutrient relocation, cell division, and tissue repair.
The plant’s internal clock is regulated by photoreceptors, such as the phytochrome system, which detects the quality and duration of light and dark. Phytochrome exists in two forms, Pr and Pfr, which convert in response to light and darkness. This conversion helps plants determine the length of the night, regulating growth patterns and processes like flowering. Continuous illumination disrupts this phytochrome cycle, interfering with natural regulatory signals.
Recommended Light Cycles for Seedlings
Most successful growers use specific light cycles that include a mandatory dark period. The goal is to maximize the light period for energy production while allowing darkness for critical metabolic processes. Recommended schedules for seedlings typically range from 16 to 18 hours of light followed by 8 to 6 hours of darkness.
The 18 hours of light and 6 hours of darkness (18/6) cycle is often considered the standard for promoting rapid, vegetative growth. A slightly shorter 16 hours of light and 8 hours of darkness (16/8) is also highly effective and may be used for plant varieties that are more sensitive to light. Maintaining a consistent schedule is important, making a reliable mechanical or digital timer essential for automation.
The specific needs of the seedling may vary depending on the plant species, but the principle of an uninterrupted dark period remains constant. A consistent dark period allows the plant’s circadian rhythm to operate smoothly, ensuring efficient energy use for strong stem and root development. Seedlings need a minimum of eight hours of dark each day to grow healthily.
Consequences of Continuous Illumination
Subjecting seedlings to continuous illumination (24/0) leads to several negative outcomes that override any potential benefit of extra light. One risk is photoinhibition, which is damage to the plant’s photosynthetic machinery caused by light stress. When the plant constantly receives light, its chloroplasts can become overwhelmed, leading to reduced photosynthetic efficiency.
This sustained stress can result in visible tissue damage, causing leaves to appear bleached, yellowed, or scorched. A lack of a dark period disrupts the energy-processing cycle, leading to reduced plant vigor and stunted growth. Energy produced during the day cannot be efficiently utilized for repair and growth without a dark phase, resulting in smaller, weaker plants.
Running grow lights for 24 hours a day is an inefficient use of resources and electricity. Since the plant reaches a saturation point where extra light no longer increases photosynthesis, the extra hours of light are wasted photons. Continuous operation also contributes to heat buildup in the growing environment. This heat can further stress young plants and potentially cause dehydration or wilting.