Sleep supports memory consolidation. Based on studies in mammals, sleep-dependent consolidation has been conceptualized as 'active system consolidation'. During waking, information is encoded into an initial store (hippocampus). During subsequent sleep, some of the newly encoded memories are selected to be reactivated and redistributed toward networks serving as long-term store (e.g., neocortex), whereby memories become transformed into more general, schema-like representations. Here we asked whether sleep in non-mammalian species might play a comparable role for memory. The literature review revealed that sleep produces enhancing effects on memory in all non-mammalian species studied. Furthermore, across species some of the hallmarking features of active system consolidation were identified: Studies of filial imprinting in chicks suggest that a redistribution of imprinting memory toward long-term storage sites occurs during sleep; song learning in birds appears to be driven by reactivations of song representations during sleep; studies of bees demonstrated the selectivity of sleep-dependent consolidation, benefiting extinction but not original classical conditioning. Although overall fragmentary, first evidence in non-mammalian species suggests active system consolidation might be an evolutionary conserved function of sleep.

Resting behavior in Aplysia fulfills all invertebrate characteristics of sleep including: (1) a specific sleep body posture, (2) preferred resting location, (3) reversible behavioral quiescence, (4) elevated arousal thresholds for sensory stimuli during sleep, and (5) compensatory sleep rebound after sleep deprivation.

Sleep supports memory consolidation as shown in mammals and invertebrates such as bees and Drosophila. Here, we show that sleep's memory function is preserved in Aplysia californica with an even simpler nervous system. Animals performed on an inhibitory conditioning task (“learning that a food is inedible”) three times, at Training, Retrieval 1, and Retrieval 2, with 17-h intervals between tests. Compared with Wake animals, remaining awake between Training and Retrieval 1, Sleep animals with undisturbed post-training sleep, performed significantly better at Retrieval 1 and 2. Control experiments testing retrieval only after ∼34 h, confirmed the consolidating effect of sleep occurring within 17 h after training.