Circadian can be translated as 'about' (circa), 'daily' (dian). Almost animals I can think of (plants, too) are responsive to the daily light/dark cycle of the sun, hence 'circadian'.
Normally, all the regulating chemicals follow one another in order like threads in a weaving pattern. Every morning the sunlight resets the cycle, stimulating the leading chemicals.
Circadian rhythm sleep disorders are dyssomnias resulting from a discrepancy between the person's daily sleep/wake patterns and demands of social activities, shift work, or travel.