The neuronal underpinnings of the sleep/wake cycle and the possible functions of sleep in cognition remain to be fully characterized. Neuromodulators such as dopamine (DA), norepinephrine (NE) and acetylcholine (ACh) have been implicated in arousal in numerous behavioral settings. Work from many laboratories, including ours, has demonstrated that the neuropeptide Hypocretin (Hcrt), also known as orexin, is essential for arousal stability, possibly by orchestrating the activity of these neuromodulators however, the neuronal mechanisms underlying this coordination are still unknown. Here, we will use transgenic, anatomical, electrophysiological, chemo and optogenetic approaches to test functional connectivity between neuronal circuits associated with sleep to wake transitions. In the first aim, we will study which hypothalamic inhibitory circuits are sensitive t external factors that affect arousal and sleep, such as acute stress and metabolic challenges, and whether these factors induce sleep by inhibiting Hcrt neurons. In aim 2 we will test the hypothesis is that DA, NE, Ach have different and specific roles on the dynamic of sleep and wakefulness and that their functional connectivity with Hcrt is critical for a healthy sleep/wake cycle. The necessity of DA or ACh transmission for Hcrt-mediated awakenings will be interrogated by photoinhibition of the neuromodulators with simultaneous photostimulation of Hcrt. In aim 3, we will test whether manipulation of individual arousal circuits during sleep has consequences on the consolidation of different types of memories. Together, these experiments will significantly increase our understanding of how Hcrt neurotransmission is modulated by external factors and how Hcrt neurons integrate and transmit this information into effector systems. Our experiments will also shed light into the mechanisms that underlie memory consolidation during sleep, with consequences in cognitive performance.