To the surprise of specialists, it was also discovered that, unlike most animals, snails do not sleep a certain number of hours per day. Their circadian rhythm is measured in two to three days. Perhaps, a clear control of the cycle of sleep and wakefulness was not developed by them, because snails need very little sleep.
Ewan Braun (zoologist, Italy) recalls that animals sleep regularly, because they need to fix new information.
The way snails live allows them to not so badly need this function. Probably, a simple form of sleep is characteristic for the early stage of evolution, and only later, when complicated life required a corresponding mental outcome, more elaborate ways of controlling circadian rhythm were developed.