State-Switching Probability Reveals Sleep-Related Biological Drives in Drosophila

ABSTRACT Sleep pressure and sleep depth are key regulators of wake and sleep. Current methods of measuring these parameters in Drosophila melanogaster have low temporal resolution and/or require disrupting sleep. Here we report a novel analysis tool for high-resolution, non-invasive measurement of sleep pressure and depth from movement data. Probability of transitioning to an active state, P(Wake), measures sleep depth while probability of transitioning to an inactive state, P(Doze), measures sleep pressure. In vivo and computational analyses show that P(Wake) and P(Doze) are independent and control the amount of total sleep. Importantly, we demonstrate that these probabilities are tied to specific biological processes. Genetic and environmental perturbations demonstrate that a given amount of sleep can be produced by many combinations of underlying P(Wake) and P(Doze). We show that measuring sleep pressure and depth continuously, without disturbing on-going behavior, provides greater mechanistic insight into behavior than measuring the amount of sleep alone.