This volume provides the first major overview, by eminent authorities on the subject, of recent developments in the field of endogenous substances and their regulation of sleep processes. The first two sections discuss general aspects of sleep regulation; including an historical overview, the restorative and adaptive functions of sleep, and evolutionary features. The third section contains contributions focussing on circadian rhythms in relation to humoral factors, hormones, neurotransmitters and metabolism. The sleep substances currently receiving most attention --- delta-sleep-inducing-peptide (DSIP), muramyl peptides, interleukin-1, sleep-promoting substance (SPS) and prostaglandin D2 --- are extensively discussed. Substances that may selectively modulate REM sleep are dealt with in the final section.