Sleep is a precious commodity. Anyone who suffers from insomnia knows how crippling sleep loss is. Richard Stephenson and Vern Lewis from the University of Toronto, Canada, explain that sleep is thought to play a pivotal role in a range of biological processes, including memory formation, but many questions still remain unanswered. For example, we don't even know how much sleep we need. According to Stephenson and Lewis, molluscs – such as the sea hare and great pond snail (Lymnaea stagnalis) – have taught us a great deal about the neural basis of memory formation, but could they teach us about the mechanisms of sleep: especially as it wasn't clear whether they do it. Stephenson and Lewis decided to find out whether snails sleep (p. 747).

‘There is no single characteristic that unequivocally defines the sleep state,’ say Stephenson and Lewis, ‘instead several criteria are used collectively.’ Describing how sleeping animals are usually unresponsive, hard to wake and settle in a characteristic position, the duo monitored the behaviour of a tank full of pond snails, cataloguing the molluscs' activities to find out whether they did anything that looked like sleep.

Analysing the snails' behaviours, Stephenson and Lewis clearly saw them attached to solid surfaces and inactive for periods of tens of minutes. And the snails looked relaxed like other sleeping species: their shells hung away from the body while attached to the side of the tank, the foot looked symmetrical and relaxed, and their tentacles were only partially extended.

Having identified a sleep-like resting state, the duo tested the snails' responses: they tapped the molluscs and stimulated their appetites when active and apparently sleeping. The scientists found that the resting snails were much slower to react than active snails, taking over twice as long to retract into their shells when poked and seven times as long to respond when their appetite was stimulated. All in all, the snails certainly seemed to be sleeping, but was their sleep rhythmic like ours and would it be affected by day length?

The team monitored the behaviour of 8 snails over 79 days as they varied the animals' light exposure. Instead of regulating their sleep over a 24 h period, the snails clustered sleep bouts in a pattern that cycled every 2–3 days. Also, they did not seem to suffer ‘sleep rebound’ – when we make up for lost sleep.

Despite the differences between mammalian and pond snail resting behaviours, Stephenson and Lewis believe that great pond snails do sleep. They say, ‘We suggest that Lymnaea stagnalis, by virtue of its anatomical simplicity and neurophysiological tractability, may prove useful in the investigation of cellular mechanisms of sleep regulation and sleep function’.

Behavioural evidence for a sleep-like quiescent state in a pulmonate mollusc, Lymnaea stagnalis (Linnaeus)
J. Exp. Biol.