I recall as kid (many years ago) seeing time lapse film of plants doing just this. Also I think there was another discussion about this samething last year.
They're just tired from working hard at vegetating and are ready for a good night sleep. All if not most plants and animals go through some form day night cycle (circadian rhythm or cycle) some are just more conspicous than others.
Beer and wine were the primery source of water throughout much of history. Alcohol kept the water safe to drink. Including on the Mayflower whiched dropped the pilgrams off at Mass instead of Vir because they were running out of ale. Can you imagine drinking stagnet water from a wood cask.