Written by: Juliann Scholl
Updated March 4, 2021
What do plants do at night? The question of whether or not plants sleep remains unanswered, and we have no way of asking the plants. Some scientists believe trees and flowers sleep because they curl up their leaves at night. Others are convinced that plants simply cannot sleep, since they don’t have a central nervous system like humans and other animals. Let’s review the arguments for and against plant sleep.
Plants Don’t Have a Central Nervous System
That plants don't have a central nervous system is perhaps the biggest argument against a plant’s ability to sleep. For humans, the central nervous system (1) plays an essential role in our sleep-wake cycles. Our breathing, blood pressure, and muscle tone all change during sleep. These changes are controlled by various mechanisms in our central nervous system.
Plants, on the other hand, have no central nervous system. If we were to say that plants “sleep,” we would have to broaden our definition of sleep.
Plants Do Tune Themselves to a 24-Hour Circadian Rhythm
As humans, our sleep-wake cycle follows an internal body clock, or circadian rhythm. This circadian rhythm runs on a roughly 24-hour schedule that mimics the patterns of the sun. When the sun’s out, we feel alert. When it goes down, we start to feel sleepy. Our circadian rhythm rules over much more than sleep and wakefulness, though. It also regulates our hormone production (2), metabolism, and body temperature, among other biological functions.
Just like us, plants also follow a circadian rhythm, which affects their behavior and physiological processes during a 24-hour cycle (3). In humans, our circadian rhythm determines when we feel hungry, energized, or tired. In plants, their circadian rhythm affects when they move their leaves (4), open and close their flowers, or release fragrances.
Plants also stop photosynthesis (5) at night . After snacking on sunlight all day long, plants dedicate their nighttime hours to metabolizing the energy they've absorbed. By the next morning, 95% of their starch reserves is converted into sugar, providing nutrition to the plant and enabling it to grow.
Plants Release a Growth Hormone During the Night
In humans, our circadian rhythm controls the production of various hormones during the day. For example, when sunlight decreases, our brain increases melatonin, a hormone that makes us sleepy. As we sleep, growth hormone (6) is released to repair our muscles and tissues from the stresses of the day, and to facilitate our growth and development during childhood.
Plants appear to release their own version of a growth hormone during the night, called auxin (7). This hormone is essential for plant growth and flower development. Whatever it is plants are doing at night, they seem to be performing some of the same functions we do during sleep.
Plants Adjust Their Circadian Rhythms to Their Environments
When our circadian rhythm is out of alignment with our external environment — whether due to jet lag or another cause — we feel grumpy, tired, unwell, and generally out of sorts. When our circadian rhythm gets back in alignment, we feel better.
The same thing happens to plants. In fact, the better aligned a plant’s circadian rhythm, the better their chance of survival (8). Studies of the flowering weed Arabidopsis thaliana show that those with better-aligned circadian clocks had more chlorophyll, grew faster, and fared better overall than their peers with mismatched clocks. A plant’s circadian rhythm can also observe and react to changes in sunlight and day length, enabling the plant to prepare for cooler weather ahead.
Plants can even adjust their circadian rhythm to defend themselves from predators, further increasing their chances of survival. Other studies of the same weed showed that the plant’s circadian clock issued the release of a foul-smelling chemical (9) right around the typical eating times of their primary predator, a cabbage looper caterpillar. As a result, the caterpillars lost their appetite, and the weed suffered less damage.
Plants can sway and move to grow closer toward light (10), but unlike humans, they can’t just get up and move to a new habitat to sleep better. Their circadian cycle enables them to watch for changes in their environment and adapt accordingly.
Some Plants Have Leaves that Curl in at Night
Charles Darwin was one of the first believers in plant sleep, noting that their leaf movement changed at night. Some modern researchers agree with his findings. The leaves and branches of European silver birch trees consistently droop by about 10 centimeters (11) during the night and slowly return to their daytime positions before sunrise. In order to avoid disrupting the plant’s circadian rhythms, researchers used infrared light to scan and measure the plant's movements.
Similarly, many flowers seem to sleep at night. For example, tulips, crocuses, and morning glories all close their flowers when it gets dark.
Some Plants Move Their Leaves During the Day
There’s one problem with viewing leaf movements as definitive proof that plants sleep. Plants can perform these leaf movements during the day, too. One group of researchers studied plant representatives from 22 different species. They found that some plants dropped their leaves during the night, while others performed leaf movements in cycles as short as two hours. Others’ leaf movements appeared completely random, suggesting that circadian rhythms in plants may be affected by more complex factors than just sunlight.
However, many animals sleep in short cycles, too, so perhaps this is evidence that plant sleep cycles can be as varied as those of animals. As one group of researchers put it, “not all trees sleep the same."
If plants do in fact sleep, they certainly sleep differently than you or I do. To observe how plants sleep for yourself, try adding some plants to your bedroom design. You may even find they help you sleep better.
- https://pubmed.ncbi.nlm.nih.gov/30155794/ Accessed on February 24, 2021.
- https://www.nigms.nih.gov/education/fact-sheets/Pages/circadian-rhythms.aspx Accessed on February 24, 2021.
- https://pubmed.ncbi.nlm.nih.gov/16595397/ Accessed on February 24, 2021
- https://pubmed.ncbi.nlm.nih.gov/29104583/ Accessed on February 24, 2021.
- https://pubmed.ncbi.nlm.nih.gov/25873925/ Accessed on February 24, 2021.
- https://pubmed.ncbi.nlm.nih.gov/8627466/ Accessed on February 24, 2021.
- https://pubmed.ncbi.nlm.nih.gov/20192736/ Accessed on February 24, 2021.
- https://pubmed.ncbi.nlm.nih.gov/16040710/ Accessed on February 24, 2021.
- https://pubmed.ncbi.nlm.nih.gov/22331878/ Accessed on February 24, 2021.
- https://pubmed.ncbi.nlm.nih.gov/22773806/Accessed on February 24, 2021.
- https://pubmed.ncbi.nlm.nih.gov/26973668/ Accessed on February 24, 2021.