People sleep (usually) at night, and dogs and cats doze pretty much whenever they can. But how about other living things, like plants? Whether or not those tulips turn in at the end of the day depends on whom you ask—and how you define sleep.
Plants don’t have a central nervous system, which is an essential regulator of sleep in humans. But they do tune themselves to a 24-hour circadian rhythm, just like you. And they do shut down certain processes, like photosynthesis, when the sun goes down, shifting their focus instead to delivering glucose (sugar) throughout the plant. Some plants, including the aptly named morning glory, also close their blooms when the sun goes down.
Sunlight is a key trigger for humans to know what time to sleep and when to be awake. And the same is true of plants. While light and dark tell your body when to produce the hormone melatonin, which cues your body to feel sleepy, light and dark triggers tell plants when to produce the hormone auxin, which controls growth and development.
Plants may not be able to get up and hunt for food (or grocery shop, or call for takeout, for that matter!), but they do move in small ways to maximize their exposure to energy-giving sunlight. During the day, they soak up the sun in order to make energy through photosynthesis, a plant’s version of eating. Then, at night, they turn their attention to metabolizing the energy that they’ve taken in and using it to grow. So maybe you should be saying “goodnight” to your garden when you turn out the lights!