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Arcady A Putilov,1,* Evgeniy G Verevkin,1,* Dmitrii V Petrovskii,2,* Lyudmila P Zakharenko2,* 1Independent Research Group “biomedical Systems Math-Modeling”, Berlin, 12489, Germany; 2Department of Insect Genetics, Institute of Cytology and Genetics of the Siberian Branch, the Russian Academy of Sciences, Novosibirsk, 630090, Russia*These authors contributed equally to this workCorrespondence: Arcady A Putilov, Independent Research Group “Biomedical Systems Math-Modeling, 11, Nipkowstr, Berlin, 12489, Germany, Tel/Fax +0049-30-53674643, Email arcady.putilov@gmail.comPurpose: Two previously proposed modelling approaches to explain the bimodal pattern of activity and/or sleep in Drosophila melanogaster are based on 1) the concept of morning and evening oscillators underlying the peaks of activity in the morning and evening, respectively, and 2) the concept of two cycles of buildup and decay of sleep pressure, gated only by the circadian oscillator. Previously, we simulated 24-h alertness–sleepiness curves in humans using a model postulating the circadian modulation of the buildup and decay phases of two (wake and sleep) homeostatic processes. Here, we tested whether a similar model could be applied to simulate the bimodal 24-h rhythm of fly locomotor activity and sleep.Methods: To obtain typical curves for the simulations, a sample of 4263 individual 24-h curves of locomotor activity and sleep were subjected to principal component analysis. It yielded three principal components, which explained more than 70% of the individual variations in these curves. We calculated the typical curves using scores on the 1st, 2nd, and 3rd principal components and simulated these curves and the sample-averaged curves.Results: We found that these curves are always characterized by two peaks with varying sizes and timings. They can be fitted by proposing the variation of some of the parameters of the two homeostatic processes reflecting the 24-h rhythmicity of the drive for wake and the 12-h rhythmicity of the drive for sleep.Conclusion: Postulation of two separate circadian oscillators is not necessary to explain the bimodal curves in Drosophila melanogaster.Plain language summary: This study tested whether a model postulating circadian modulation of the buildup and decay phases of two homeostatic processes (wake and sleep) can be applied to simulate the bimodal 24-h rhythm of fruit fly activity and sleep. The simulations suggested that the typical bimodal curves can be simulated by proposing variation of some of the parameters of the two underlying homeostatic processes that represent the 24-h variation in the drive for wakefulness and the 12-h variation in the opposing drive for sleep.Keywords: sleep–wake regulating process, bimodal 24-h rhythm, fruit fly, math-modelling

Lyudmila P Zakharenko,1,2 Dmitrii V Petrovskii,1 Arcady A Putilov3 1Department of Insect Genetics, Institute of Cytology and Genetics of the Siberian Branch, the Russian Academy of Sciences, Novosibirsk, Russia; 2Faculty of Natural Science, Novosibirsk State University, Novosibirsk, Russia; 3Research Group for Biomedical Systems Modeling, Research Institute for Molecular Biology and Biophysics, Novosibirsk, Russia Purpose: Drosophila melanogaster and our own species share (Homo sapiens) the history of relatively rapid out-of-Africa dispersal. In Eurasia, they had faced a novel adaptive problem of adjustment of their circadian rhythmicity and night sleep episode to seasonal variation in day length and air temperature. Both species usually respond to heat and a short duration of night by reduction of the amount of night sleep and prolongation of “siesta”. To further explore similarities between the two species in the ways of adjustment of their sleep–wake behavior to extreme environmental factors, this study examined the possibility to distinguish four extreme chronotypes among fruit flies and the possibility of the differential response of such chronotypes to light and heat stressors. Materials and methods: Circadian rhythms of locomotor activity and sleep–wake pattern were tested in constant darkness, and four strains of fruit flies originating from three wild populations of Africa, Europe, and the USA were selected to represent four distinct chronotypes: “larks” (early morning and evening activity peaks), “owls” (late morning and evening peaks), “swifts” (early morning and late evening peaks), and “woodcocks” (late morning and early evening peaks). The circadian rhythms and sleep efficiency of the selected chronotypes were further tested under such extreme conditions as either long day (LD20:4 at 20°C) or a combination of LD20:4 with hot temperature (29°C). Results: Despite the identity of such experimental conditions for four chronotypes, their circadian rhythms and sleep timing showed significantly distinct patterns of response to exposure to heat and/or long days. All two-way repeated measures analysis of variances yielded a significant interaction between chronotype and time of the day (P