Do trees sleep? It certainly doesn’t seem that way. But as odd as it may sound, trees really do sleep just like any other plants or animals, and scientists have seen them resting their branches while asleep for the first time.

Researcher András Zlinszky of the Center for Ecological Research in Tihany, Hungary and his colleagues used laser beams to scan trees from sunset to sunrise in two different countries, Austria and Finland. Measuring the time it takes beams to bounce back from branches and leaves, they could confirm that branches of trees drooped by as much as 10 centimeters at the tips towards the end of the night.

“It was a very clear effect, and applied to the whole tree,” András Zlinszky told New Scientist. “No one has observed this effect before at the scale of whole trees, and I was surprised by the extent of the changes.”

To make sure the movement of the trees was not caused by wind, researchers conducted laser scanning on calm nights, and at the solar equinox in both countries so that the length of night roughly stays the same.

Trees Sleep At Night
Tree branches droop more at night (left) than during the day (right), indicating that they sleep at night. Image: Vienna University of Technology, TU Vienna

The drooping effect or the phenomenon in which branches of trees move by its own is caused by loss of internal water pressure (called turgor pressure) within plants cells, which means branches and leaf stems become less rigid and more prone to drooping under their own weight at calm hour at night.

Moreover, since turgor pressure is influenced by photosynthesis, Zlinsky thinks the branches droop because plants cease to carry out photosynthesis at night. He also thinks that the trees might be sleeping after a long day in the sun – being tired of having their branches and leaves angled higher to catch more sunlight all day.

“The experiment is the first of its kind,” says Eetu Puttonen, a team member. “These studies have only been done before in small plants, but here, it was possible to do it outside in fully grown trees.”

Is drooping deliberate act of plant influenced by an active sleep-night cycle, or by differences in the availability of water and light? The answer is yet to be decided. However, it should answer – “Why do trees make strange and scary noises in the night?”