Aestivation is a period of deep and prolonged sleep.
This occurs in the summer or the dry season, food can become scarce so animals ( snails in this case) avoid using their energy reserves.
These snails Cernuella virgata also known as Helicella vulgata (the vineyard snail) easitvate on the tops of plants and fences.
This snail was introduced to Britain by the Romans, probably with weeds.