What trees do at night may not have crossed your mind, but a team of scientists from Finland, Austria and Hungary has made a fascinating discovery on the topic. According to a report from the Washington Post, researchers have shown that certain trees do in fact “sleep,” at night.

Scientists used lasers to track the movements of a number of trees during the night hours, and were surprised to find that many in the sample appeared to relax and droop by as much as four inches when it became dark outside.

According to Andras Zlinsky, a researcher with Hungary’s Centre for Ecological Research, “It was a very clear effect and applied to the whole tree. No one has observed this effect before at the scale of whole trees, and I was surprised by the extent of the changes.”

Scientists honed in on two sample birch trees, one in Finland and one in Austria. They scanned the canopies of the trees with lasers from sundown to sunrise over regular intervals for the course of one night. Each tree was under similar environmental conditions during the test – light wind, no rain, and around the time of the solar equinox, meaning there were a similar number of light and dark hours.

Scientists wrote, “The results obtained from the measurements showed that the crown movement in the Finnish and the branch movement in the Austrian study case presented similar temporal response. Close to sunrise, the branches were hanging lower than at the time of sunset… The movements were observed to happen systematically over a time span of several hours, which ruled out occasional wind effects.”

Researchers aren’t quite sure why trees go to sleep at night, but they believe it could be linked to the circadian rhythms observed in a number of other species. “The fact that some branches started returning to their daytime position already before sunrise,” wrote the authors, suggested that the tree is “governed by the internal circadian clock of the plant.”

A press release from the National Land Survey of Finland describing the details of the study can be found here.