Foliar Nictinasty (FN) is the tendency for (some) plants to open their leaves during daylight hours and close them at night - tightly following the day/night cycle.

This process has been observed and debated for at least 2,000 years. In 1729 Linnaeus suggested that the plants were 'sleeping'. Later, Charles Darwin compiled a list of 79 plants which show FN.

Although many of the bio-mechanical mechanisms which control the movements have been identified, modern-day botanists don't as yet have an agreed explanation as to why the opening/closing cycle has evolved - or what benefits it may have for the plant.

Theories include :

• Reducing nocturnal transpiration
• Reducing the leaching of minerals from leaves
• Reducing the need for leaf support tissue
• Preventing the growth of epiphylls and fungal pathogens
• Reducing moonlight-induced disruption of photoperiodism
• Reducing nocturnal herbivore activity

- and many more. See : The functions of foliar nyctinasty: a review and hypothesis Biological Reviews, Volume 94, Issue 1 p. 216-229

Note: Complicating the picture, the majority of plants have little or no tendency for FN. And some species , e.g. the Prayer Plant (Maranta leuconeura) do the exact opposite - opening their leaves at night, and closing them during the day.