If you've ever taken a late-night stroll through a garden, you may have noticed that certain flowers, much like people, tend to retire after the sun goes down.
But flowers that close up at night, such as tulips, hibiscus, poppies and crocuses, aren't sleepy. They're just highly evolved.
Plants that tuck themselves in for bedtime exhibit a natural behavior known as nyctinasty. Scientists know the mechanism behind the phenomenon: In cool air and darkness, the bottom-most petals of certain flowers grow at a faster rate than the upper-most petals, forcing the flowers shut.
For the rest of the story: http://www.livescience.com/34569-why-flowers-close-at-night-nyctinasty.html