FIG. 1.

A hierarchy of rhythms in mammalian systems. Circadian rhythms are observed at every level of biological scale with causation acting in both directions. Appropriate environmental cues can evoke changes of rhythmic gene expression and cellular processes, whereas changes in gene expression and cellular activity ultimately regulate behavioral phenotypes such as the sleep/wake cycle. Unlike most mammals, humans have direct control over many environmental factors, for example, electrical lighting, adding an additional layer of complexity. Diagram inspired by Noble (97).