As weird as the question may be, do plants sleep? (imagining a tree feeling dizzy, its head going forth and back, uhn, Hell No).
I mustn’t be too quick to answer a yes or a no here. Although plants don’t have central nervous systems that seem to be key in what we think of as sleep in humans, plants do have circadian rhythms tuned to Earth’s 24-hour light-dark cycle, which they maintain even if they’re kept in light fulltime, just as we do. And that is where things get really interesting.
For us, the circadian cycle determines when we should sleep and when we should wake up: sunlight enters our eyes each morning, triggering cells in the brain that control levels of the hormone melatonin, which, in part, controls drowsiness. The more melatonin, the sleepier we are. Melatonin levels drop in the daytime and rise at night. And while our main sleep clock resides in the brain, we also have clock genes in nearly all of the cell types throughout the body, and vital physiological processes occur as we sleep.
According to a plant biologist at Rice University, plants also go through physiological changes during each stage of the day.
Indeed, plant behavior is tightly controlled by the sun. During the day, plants soak up sunlight during photosynthesis, to gain energy. But when the sun goes down, plants’ opportunity to eat disappears and other physiological processes take over, including energy metabolism and growth. Plants can anticipate the dawn each day and follow the sun to maximize their photosynthesis potential. The sunflower, for example, sways back and forth as the sun rises and falls, and it’s seedlings appear to dance as they reach for sunlight. Also, corn seedlings have been seen to bow towards a light bulb.
These researchers have also found that the circadian rhythm in certain plants also determines when they launch chemical defenses against predators.
A 2012 study on Arabidopsis, a highly studied flowering plant related to cabbage, found that the plant’s circadian cycle helps ward off cabbage looper caterpillars. A set of plants kept on a normal day/night cycle anticipated the time the caterpillars typically eat and gave off a pungent chemical to discourage feeding. Plants forced on a light cycle 12 hours out of phase didn’t do this and were thus chewed up.
And in a study from last June, the researchers showed that cabbage retains its circadian rhythm after harvest, including the cyclical production of a chemical called 4MSO, which may have anti-cancer properties.
So just like the microbes, the plants run a circadian cycle. Oops!.