:For years we had this idea that insomnia is an abnormality of the sleep system. Increasingly, we have learned that insomnia is not an abnormality of the sleep system, but it is an abnormality of the wake/arousal system. Over a decade ago, Saper and colleagues proposed the flip-flop switch model of sleep-wake regulation,6which contains 2 sets of mutually inhibitory neural elements: wake-promoting influences on 1 side and sleep-promoting influences on the other. The monoaminergic nuclei (MN) are a major influence of the wake-promoting system, and sleep is influenced by the ventrolateral preoptic nucleus (VLPO), which is a group of cells that generate non-rapid eye movement (NREM) and rapid eye movement (REM) stages of sleep (Figure 2).5In patients with normal sleep, the flip-flop switch makes sudden transitions between sleep and wakefulness, which explains why a limited amount of time is spent in these transitional states throughout the course of a 24-hour day (Figure 3).6In patients with insomnia, however, their ability to turn off the wake-promoting influences of the flip-flop switch is weakened, resulting in extended periods of time in transitional states and a prolonged state of wake.