The next time you decide to take a midnight stroll through the forest, mind your footsteps. The trees are sleeping.

That's the fascinating conclusion drawn by a team of scientists from Austria, Finland and Hungary who wanted to know if trees followed day/night cycles similar to those observed in small plants. Using laser scanners pointed at two birch trees, the scientists recorded physical changes indicative of a nighttime slumber, with the tips of the birch branches drooping by as much as 4 inches towards the end of the night.

"Our results show that the whole tree droops during night which can be seen as position change in leaves and branches," Eetu Puttonen from the Finnish Geospatial Research Institute said in a statement. "The changes are not too large, only up to 10 cm for trees with a height of about 5 meters, but they were systematic and well within the accuracy of our instruments."

birch tree sleeping A birch tree at night (left) experiences more branch dropping than during the day (right). (Photo: Eetu Puttonen/Vienna University of Technology, TU Vienna)

In a paper published this month in Frontiers in Plant Science, the scientists explained how they scanned two trees, one in Finland and another in Austria. Both trees were scanned independently, on calm nights, and around the solar equinox to ensure a similar length of night. While the tree's branches were shown to droop lowest just before dawn, they returned to their original position in only a few hours.

The researchers believe the dropping effect is caused by a decrease in the tree's internal water pressure, a phenomenon known as turgor pressure. With no photosynthesis at night to drive the conversion of sunlight into simple sugars, trees likely conserve energy by relaxing branches that would otherwise be angled towards the sun.

“It was a very clear effect, and applied to the whole tree,” András Zlinszky of the Centre for Ecological Research in Tihany, Hungary, 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.”

The team will next turn their lasers on other forest species to see if they too exhibit a circadian rhythm. “I’m confident it will apply to other trees,” Zlinszky added.