Why the scientific finding that trees “sleep” at night is beautiful

From the tiniest bacteria to the largest redwood tree, almost all life on planet Earth is ruled by the 24-hour clock.

Genes that code for a 24-hour circadian rhythm exist “in most every cell, in most every organism on Earth,” Michael Twery, who leads sleep research at the National Institutes of Health, told me in 2015. The activity-rest cycle is fundamental to life, he said.

It shouldn’t be surprising: Life evolved against the backdrop of sunrises and sunsets.

But there’s an awesome beauty in it — it’s like the whole natural world is humming along to the same beat. It means our experience of day and night, sleep and wakefulness, is not just a human quirk but a sign we’re part of the natural world too.

Scientists are finding evidence of biological clocks in all sorts of creatures. In fruit flies, in mice, and in single-cell organisms (which can, like us, get jet lag). And here’s one more, from the journal Frontiers in Plant Science: Birch trees “sleep” at night.

The researchers pointed laser scanners at two birch trees, one in Austria, and one in Finland. The lasers bounced light off the tree as sensors detected small changes in its shape.

As the night wore on, the branches drooped as much as 10 centimeters, as if they were taking a load off from a long day. In other words, the tree seemed to relax for a few hours each night, specifically about two hours after sunset until just before sunrise, “after which the tree activates again,” Eetu Puttonen, one of the study authors, writes me in an email. (You can see the nightly movement of the Finnish tree in the animation above.)

While study is the first to find “spatial changes in tree branch geometry over a day-long cycle,” according to the paper, Puttonen notes that the findings are still preliminary. It’s just one tree species, and the study actually just meant to test the effectiveness of using such a laser system to measure plant circadian rhythms.

Researchers have been able to measure the rhythms of lots of smaller plants in the lab. (Darwin himself noted that plant leaves “sleep” at night.) But it’s harder to do with big trees in the forest. Using the technique developed by Puttonen and his co-authors, it could be possible to see a whole forest winding down its branches at night.

Why are the trees drooping? One reason could be that they’re dropping their internal water pressure, as New Scientist suggests, in response to the cessation of photosynthesis at night. Another reason could be that the trees are actually resting: It takes energy to raise limbs up toward the light during the day.

The researchers note that they were careful to choose a windless night at each location, and that each measurement was conducted near the autumnal equinox in September, to control for differences in daylight.

Puttonen hopes this research will lead to a better understanding of how trees use water at different times of the days. That might “help the timber industry (where felled trees should preferably have a low water content), and the rubber industry (where the sap of the tree is exploited and high tree water content is desired during harvesting),” he writes. Since the study published in 2016, a followup report in 2017 found that different species of trees have different “sleep” patterns. So more research will be needed to full understand the complex patterns of tree sleep.

Correction: This post originally confused the vernal equinox for the autumnal equinox.