Here’s my plagiarized response:
Yes. Cannabis plants, like most plants, take in water through their roots, use that water in a variety of chemical reactions, and exhale the byproducts (namely oxygen as O2) through microscopic holes in their leaves called stomata.
During the day these stomata open to allow respiration to occur. If the plant can take in slightly more water through its roots than it can breathe out through its leaves the plant becomes turgid, or filled with water to the point that it becomes stiff. This is because osmosis is ultimately driven by evaporation.
Think of the plant as a drinking straw filled with liquid. At the top of the plant there is a device that allows the water to evaporate without losing the negative internal pressure inside the straw that is drawing the liquid up. As the liquid evaporates, more is drawn through the straw. This is the process plants use to supply themselves with water. It is a physical, mechanical process that, once set in motion, will not stop without intervention, although it can be regulated by the plant through a variety of methods.
Turgidity in plants is a great indicator of a plant's health. Unhealthy plants often have a very hard time maintaining this ratio. If the air is too dry and water evaporates too quickly for the roots to keep up plants will close their stomata to slow down the rate of evaporation in an effort to maintain turgidity. If the air becomes even drier the plants will begin to wilt as gases leave the plant faster than it can replace them with liquids.
At night the plants have no reason to respire. There is no sunlight to drive the processes that require respiration, so the plant essentially closes shop. Its stomata shut almost entirely to preserve water and its roots cease to uptake water and nutrients because the driving force (evaporation) can no longer coax these resources up the stalk. Since it can't do real work at night, the plant uses this time to rest. It is akin to sleep in that the plant is able to repair itself from the work done during the day. Some physiological process do proceed. Some water and nutrients continue to be used at a much slower rate than during the day.
Because the plant is still using water and nutrients, some byproducts get exhaled through the almost-totally-closed stomata and cannot be replaced through osmosis as they would during the day. This means that the water holding the leaves in a “praying" position during the day gets depleted, causing the plant's internal turgidity to drop and in turn causing the leaves to droop.
It is normal and healthy for this cycle to continue in this way through the life of the plant. Drooping at night is expected, but if the plants start dropping during the day it is likely an early indicator that they aren't growing at optimal efficiency and the growing environment or soil nutrients could be less than ideal
