Circadian rhythms are basic biological phenomena that exist throughout phylogeny. Circadian clocks oscillate with daily periodicity and regulate many physiological conditions and behaviors that include the sleep-wake cycle, metabolism, body temperature and feeding. Learning and memory efficiencies depend on the time of day in various organisms, but it was not clear whether and how the circadian clock regulates memory performance. Recent studies in both vertebrate and invertebrate creatures have shown circadian-dependent effects on learning and memory. Genetic or surgical disruption of circadian clock has negative consequences on memory performance in various memory tasks across several species. In Aplysia, fruit fly, cockroach, zebrafish, and rodents, circadian fluctuations in learning and/or memory have been observed under constant dark condition, while their peaking times in the performance are diverse among the organisms and/or the experimental paradigms. Here I summarize studies that have shown a circadian effect on memory performance and compare the common themes in these invertebrate and vertebrate species. I also describe our new discoveries that have shown SCOP-dependent molecular mechanism of circadian regulation of recognition memory.

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To maintain a healthy life, organisms need to feed appropriate nutrients. Animals utilise a variety of sensory systems; i.e., gustation, olfaction and vision; to detect and obtain required nutrients from the ambient environment. A sweet or fatty taste is a reward for hungry animals and induces feeding responses, however; an excess of such a nutrient causes problems such as obesity or diabetes. Therefore animals have to make a decision on what, when and how much to eat. Such a decision-making process on a feeding behaviour exists in a wide range of taxa, including mammals and invertebrates. Although insects have a simple nervous system compared with mammals, mammals and insects share many common physiological systems and have a degree of genetic homology, making insects great models for the genetic and neurobiological studies on decision-making. Here I introduce novel findings on the decision-making on feeding behaviour in insects, mainly fruit flies.

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