I have some tropical and subtropical hibiscus that fold their leaves down to a more or less vertical position in the evenings. My Hibiscus cubensis shows this trait to the most extreme. I also have a Hibiscus mutabilis from American Samoa that folds its leaves down somewhat at dusk. My Hibiscus dasycalyx and its hybrid progeny also show some tendency to fold their leaves at night. My theory is that it could be a protective mechanism against heavy nighttime showers in the tropics and subtropics. Anyone familiar with this phenomenon?