We spend about a third of our lives asleep, so you'd think we'd have a pretty good understanding of it by now, but there’s still a great deal about sleep that scientists don’t get. What purpose do dreams serve? Why do humans and other animals need to sleep at all? And what about plants? Well, a new study suggests that even trees get their snooze on at night. The report was published in the journal Frontiers in Plant Science. 

The concept of bedtime for plants is not new; Charles Darwin himself addressed the subject in his 1880 book The Power of Movement in Plants. Darwin and his son/co-author Francis acknowledged the sleep-like curling of flower petals and the turning of leaves after dark, but were careful to distinguish this action from human rest, writing, “Hardly any one [sic] supposes that there is any real analogy between the sleep of animals and that of plants …” Many researchers have studied the phenomenon since, watching for hours as leaves and spines made their infinitesimal transitions in the dark. Yet all of those studies involved potted plants, and none of them included trees. 

So researchers set up a small study of two silver birch trees, one in a forest in Finland and one in Austria. Rather than relying on naked-eye observation, the biologists scaffolded the trees with laser scanners that recorded the trees’ movements in the night. The millions of points of data from those scans were then compiled into tree-shaped clouds. 

Puttonen et al. in Frontiers in Plant Science

Despite a separation of more than a thousand miles, the two trees behaved quite similarly. After sunset, the birches’ branches began to droop, and continued to do so for several hours. Then, as sunrise approached, the branches slowly rose again until they had reached their original position. 

"The changes are not too large, only up to 10 cm for trees with a height of about 5 meters,” co-author Eetu Puttonen of the Finnish Geospatial Research Institute said in a press statement, “but they were systematic and well within the accuracy of our instruments." 

Whether the trees are roused by sunlight, an internal clock, or some combination of both is still unknown. 

This is the first study to use terrestrial laser scanning (TLS) to monitor plant activity, and it offers unique insights into the processes behind the action. "Plant movement is always closely connected with the water balance of individual cells, which is affected by the availability of light through photosynthesis,” said co-author András Zlinszky of the Centre for Ecological Research, Hungarian Academy of Sciences. “But changes in the shape of the plant are difficult to document even for small herbs as classical photography uses visible light that interferes with the sleep movement." 

The researchers say the high-fidelity results produced by TLS could also be very effective for studying groups of trees in orchards or forests.