do snails sleep and for how long?

i have a 20l tank with a couple of gouramis and a red eye tetra, they are going brilliantly waiting for their 150l tank to cycle. All very active, growing and best colours. However, my 2 mystery snails have been at the top for a few hours. They look a good colour but they are not moving, even when I emptied some of the tank. Nice helpful answers please...:)