Sleep is a ubiquitous behavior observed across animal species. Sleep in mammals has two distinct states, rapid-eye-movement sleep (REMS) and slow-wave sleep (SWS)/non-REM sleep, which have different homeostatic regulations and neural dynamics. In mammalian species, sleep has a relatively clear definition based on electroencephalography (EEG) and electromyography (EMG) that allows us to evaluate wakefulness, SWS, and REMS status. However, sleep research on non-mammalian and invertebrates is largely based on behavioral criteria. To investigate further the evolutional origin and individual roles of different neural states and oscillations, in this Grass Fellowship program, I particularly focus on the naturalistic behaviors in amphibians. I develop a framework of long-term in vivo electrophysiological recording in freely behaving frogs. Combining in vivo electrophysiology, molecular biology, and behavioral and computational analysis, I investigate the evolutionally conserved mechanisms underlying sleep/wake behaviors and state-specific neural representations. This project may provide insights and various applications in behavioral, evolutionary, and ethological neuroscience.