Sleep homeostasis: Progress at a snail's pace.

We recently reported that a gastropod mollusc, Lymnaea stagnalis, exhibits a sleep-like quiescent state. However quiescence differed from mammalian sleep in that both circadian and homeostatic regulation seemed to be absent in the snail. In the present paper I explore the possibility that the clustered pattern of quiescence observed in the snail may provide insight into underlying regulatory mechanisms. Specifically, I hypothesize that clustering of quiescence arises from feedback modulation of a stochastic wake-quiescence oscillator. The feedback loop is postulated to limit cumulative wakefulness and to exert probabilistic modulation of wake duration. Computer simulations using this model succeeded in simulating snail wake-quiescence behaviour, confirming the plausibility of the hypothesis. Implications of this analysis for our understanding of mechanisms and evolution of sleep homeostasis are discussed.