This work proceeds from a deduction from evolutionary principles, that primitive sleep was non-uniform preceding the origin of the REM and NREM states. Coupling this conclusion with knowledge of the thermoregulatory responses to ambient temperatures by sleeping mammals, and indications that the REM and NREM sleep states originated during the evolution of warm-bloodedness in the Mid-to-Late Triassic period, implicates ambient temperature as the environmental variable responsible for the non-uniform condition of primitive sleep. Drawing on the paleontology and paleoecology of early ancestors of mammals, together with the uniquely warm and moist, moderate climates of the times, a sequence of selective pressures and resulting adaptations are proposed, leading to the origin of REM and NREM sleep and a basis for their cyclic alternation. (Sleep and Hypnosis 2002;4(3):85-92)