Ultradian Rhythms from Molecules to Mind5. 1. 1 Biological Rhythms and Clocks From an evolutionary perspective, the adaptation of an organisms behavior to its environment has depended on one of lifes fundamental traits: biological rhythm generation. In virtually all light sensitive organisms from cyanobacteria to humans, biological clocks adapt cyclic physiology to geophysical time with time keeping properties in the circadian (24 h), ultradian (24 h) domains (Edmunds, 1988; Lloyd, 1998;