Plants, like all living creatures follow a circadian rhythm. This rhythm follows a roughly 24-hour cycle and is stimulated, not by outside triggers, but by a plant’s own DNA. Whether a plant light is left on or off a plant still experiences its life in 24-hour cycles, to include sleeping and waking cycles. One night with the lights on will not usually harm a plant. It is important, however, to consider the plant species, the type of light being provided and other growing conditions in order to lessen any potential risk to the plant.
Incandescent lights provide heat as well as light. If your plant light is incandescent then be certain to provide your plant with extra water and monitor it for damage. Fluorescent lights, both the warm and cool varieties should not cause any damage to the plant if left on for 24 hours.
If you habitually forget to turn the light on or off then consider purchasing a grow light with an automatic timer. The timers alone are usually inexpensive but most grow light kits come with included timers.
Most plants fall into one of the three categories, according to Oregon State University: short-day plants, long-day plants and day-neutral plants. Short day plants require long periods of uninterrupted darkness in order to produce blooms. Long day plants require short periods and day neutral plants do not have specific darkness requirements.
Examples of short day plants are plants which normally bloom in early spring and in the autumn, when days are naturally shorter--12 hours of daylight or less. Think of chrysanthemums, Christmas cactus and poinsettias. These plants only bloom when they receive 12 hours of sunlight or less. Gardeners and nurseries often shade these plants for part of the day so that they bloom at the optimum time for sales and winter holiday decorations.
Long day plants are plants that require less darkness and more light. Daisies, poppies and lettuce fall into the long day category. Again, in order to speed the rate of bloom gardeners and nurseries often use artificial light to extend natural winter days in order to have the plants available for sale or ornamentation.
Examples of day neutral plants, which do not have specific requirements, are petunias and corn.
In the 1930s, researcher Francis Ramaley reported in Botanical Gazette the results of an experiment that was conducted on 100 species of plants that received 24-hour lighting for an entire winter. Many of the plants exhibited straggly, pale growth and weakened stem and root structures. Early blooming occurred with many temperate zone plants but tropical plants did not see much difference.