Planted Tank Guru
Join Date: Jun 2004
Location: Baton Rouge, LA
Plants tend to operate in one of two modes.
When the lights are on, plants grow very little, instead devoting their attention to photosynthesis and storing chemical energy. Imagine they have a "gas tank", like a car, that they're filling up.
When the lights are off, they devote their attention to growing using the energy they've stored away in their gas tank.
The capacity of the gas tank is limited, but it's often enough for several days of growth. It's common to see an impressive burst of plant growth if you skip a light cycle or two.
With brighter lights, the gas tank fills up faster. If the lights are on for 12 hours instead of 8, that means the plants have four hours less dark time each day to actually grow and use what's in their gas tank.
And when the lights come back on, the gas tank is still rather full, so it tops off in just a few hours. The rest of the time the lights are on is wasted as far as the plants are concerned; since they can neither gain energy nor grow during that time.
Algae, on the other hand, seems to be able to photosynthesize and grow at the same time; so they love long, bright photoperiods.
That's a simplified explanation of why we shorten the photoperiod a few hours when using bright lights.