The chapter will summarize our current understanding of the neuronal and neurochemical basis of hypnogenesis. The hypothesis of the localization of a hypnogenic mechanism in the mammalian hypothalamic preoptic area (POA) was first proposed by von Economo more than 70 years ago (von Economo, 1930). This hypothesis has been confirmed by findings that experimental POA lesions suppress sleep, and that electrical, chemical, and thermal POA stimulation induce sleep (reviewed by McGinty & Szymusiak, 2001). Unit recording studies have identified POA neurons that exhibit increased activity during NREM sleep, REM sleep, or both. These sleep-active neurons are hypothesized to be the substrate of the hypnogenic mechanism. The past decade has seen substantial progress in the further description of this hypnogenic system; we summarize this progress in this chapter.

Localization of sleep-active neurons within the POA

Studies of sleep-active neuronal discharge across the sleep–wake cycle in freely moving animals provide important information about the hypnogenic process (see below) but, because of sampling limitations, are not suitable for systematic mapping of the exact locations of putative hypnogenic neurons. The application of the c-Fos immunoreactivity (IR) method to map sleep-active neurons has stimulated several advances. C-Fos IR is a marker of neuronal activation in most brain sites; immunohistochemically labeled neurons can be mapped systematically.