The book Why We Sleep mentioned an experiment. Some plants obviously change during day or night, extending leaves during the day and letting them droop at night, something like that from what I remember. People thought the plant was just reacting to the sunlight, until they put the plant in total darkness for several days and observed it going through the same cycles. The plant was not only responding to light, but also to an internal timer. Depending on how you define sleep, this is sleep. Granted plants don't use sleep for all the same purposes as animals.

If you find that article, I'd love to see it. I know a bit about plant hormones, but most of what I remember is some of the growth hormones degrade in sunlight, which can cause the apparent growth toward the sun. So if you have a stem and the light is coming from the right, the cells on the right side don't divide or elongate as quickly as the cells on the left side in the shade, which causes the stem to bend to the right. This is also the cause of etiolation, or legginess, in plants that aren't getting enough sun. Those cells have plenty of the hormone as it's not being degraded by the sunlight, causing them to elongate more quickly than a plant getting adequate light.

"In 1729, French scientist Jean-Jacques d'Ortous de Mairan conducted the first experiment designed to distinguish an endogenous clock from responses to daily stimuli. He noted that 24-hour patterns in the movement of the leaves of the plant Mimosa pudica persisted even when the plants were kept in constant darkness."

https://en.wikipedia.org/wiki/Circadian_rhythm#History second paragraph

Awesome! Thanks!

I don't know if you could call it "sleep" per se, but there are certainly plenty of plants that need a daily period of darkness to grow properly. This comes up when growing indoor plants when people say you shouldn't run a grow light 24/7.

Plants increase their respiration at night and stop photosynthesizing.

https://www.bbc.co.uk/bitesize/guides/zxtcwmn/revision/1

It seems kind of like sleeping to me.

One difference at night is that it is cooler, so less water may be lost when the stomata are opened for gas exchange. Many plants (separately) evolved a photosynthesis cycle where gases need only be exchanged at night and the stomata may be closed during the day.[1] While the actual photosynthesis reaction happens during the day, the nighttime preparation is needed, and should perhaps be counted towards the effort.

https://en.m.wikipedia.org/wiki/Crassulacean_acid_metabolism

Although without sunlight, they have no choice but to stop photosynthesizing.

Then we have plants like the dracaena fragans, whose flowers remain closed at day, but open and release their delicious fragrance all night. I'm not sure how sleep would work for her.

https://en.wikipedia.org/wiki/Dracaena_fragrans

Daytime sleeping is not unheard of.

I mean, that's like 99.9999....% expected, since they all evolved with a big light signal in the sky that operates on a 24 hr cycle. Plants, even more than animals, should be optimized to function that way (do mostly resource production at day, do mostly maintenance at night).

Nitpicking: "stop photosynthesizing" is just a direct consequence of the lack of ambient light. People stop getting sunburn at night, that's not part of "sleeping".

Other than that: +1.

Plants physiology changes significantly during the night, so it's not just "no more photosyntheses without Sun"... respiration rates at night are strongly correlated with the amount of stored carbon and sucrose in the leaf tissue, which is probably related to the fact that plants also grow faster at night.

Maybe not.

Plants have two sources of nutrition: uptake via their roots and photosynthesis. My general understanding is that photosynthesis is what provides them energy whereas uptake via roots provides water and some essential nutrients they can't synthesize.

The details will vary and some plants, for example, grow in total darkness or feed parasitically on other plants. But I think photosynthesis is roughly the equivalent of humans eating carbs for energy.

So it's a little like saying "One defining characteristic of sleep is that animals stop eating." And, in fact, hibernation through the winter when food is in very limited supply is a known mechanism of survival for some species in harsh climates: They sleep instead of eating.

In fact, my understanding is that to some degree eating and sleeping are substitutable. If you can't sleep, you can eat to help keep yourself functional while awake on a long shift.

I think they do this on episodes of Deadliest Catch: feed people more when they are pulling a long shift and going to be awake for 24 hours straight.

Well, you raise a non-trivial point that is more than just a semantic issue, viz what do we mean by "sleep"?

According to this plants can tell time.

https://youtu.be/CA-YsWXRSHU?t=453

There was a paper, I think it was also linked on HN via quanta magazine, where some researchers seemed to have pinpointed the reason why sleep is necessary to live, at least for animals and insects.

However, it seemed too good to be true (I can only wait to see if there is a Nobel prize) and it's not clear to me what to make of it.

The gist of it was that the fundamental damage from lack of sleep actually originates in the gut, not the brain, across rather different species, (mice and fruit flies) where "reactive oxygen species" (iirc) cause cumulative damage that is eventually fatal.

The researchers claimed they had confirmed causality by reversing the shortened lifespan in fruit flies deprived of sleep by feeding them antioxidants.

I have never seen a paper that seemed to so thoroughly eliminate the possibility of a misleading correlation, but the sticking point for me is, if it is so simple to avoid the need for sleep, why hasn't it been eliminated by evolution in the first place?

(Sleep Loss Can Cause Death through Accumulation of Reactive Oxygen Species in the Gut Vaccaro, Alexandra et al. Cell, Volume 181, Issue 6, 1307)

Plants also have nyctinasty which isn't sleep but is interesting: https://www.livescience.com/34569-why-flowers-close-at-night...

Anesthesia works on humans too, and we still don’t know why.