Sleep structure and quantity are determined by behavioral transition probability in Drosophila melanogaster

In humans and flies, sleep is a highly organized behavior, occurring in discrete episodes that are consolidated at night but have a more fragmented structure during the day. Longer episodes correlate with transitions to deeper sleep states for both species, and sleep fragmentation has been shown to have deleterious outcomes for a large number of physiological processes. In mammals and birds, EEG provides an excellent measure of sleep structure and state. In invertebrates, assessment of sleep structure has relied on measures of episode duration, which can fail to capture important aspects of sleep architecture. Here we model Drosophila sleep as a probabilistic behavior and develop analytical tools to measure transition probabilities and the organization of sleep episodes within a defined analysis window. We show that these tools provide insight into the underlying mechanisms of normal sleep and into how they are altered in pathological conditions. The probability of transition to the wake state, P(Wake), is a major driver of normal sleep quantity and is responsive to dopaminergic tone, suggesting it is a measure of arousal. The probability of transition to the sleep state, P(Doze), is regulated by sleep deprivation and contributes significantly to regulating sleep fragmentation, suggesting it is a measure of sleep drive. Our method for assessing sleep fragmentation adds important information about the organization of sleep bouts that is missing in measures of episode duration alone. Importantly, the ability to measure behavioral drives provides a way to compare the effects of neural circuits and perturbations that play a role in more than one sleep-relevant process.