I started finding chocolate for my snail. It says that chocolate makes your snail gather coins faster in your Zen garden, but how long do the effects of chocolate last? Also, can I feed my snail multiple chocolates to make the effect last longer? It would be cool if I could feed my snail enough to work while I sleep.
As weird as it is, you can also feed chocolate to your plants, causing them to drop coins more quickly.
You say you would like to leave the Zen Garden open while you sleep, however I don't know if that's a good idea:
I don't know what happens to the Zen Garden right after the midnight. I know that closing the game before the midnight and opening it after the midnight will make the Zen Garden "reset" to all plants needing watering. I don't know if the same happens if the game is left open during the midnight.
Finally, the same page from that wiki says that you can move the mushroom and aquatic plants into the main garden section (if they are already happy/glowing), and they will still drop coins even in the "wrong" environment. Read that page for more details.
According to the wiki, the effects of the chocolate last for one hour. I am pretty sure that it does not stack, so feeding it chocolate at any time will just keep it fast for an hour after the feeding.