Diurnal rhythm of life - repetitive actions developed during the formation of the species as an adaptation primarily for food consumption and defense against enemies.

The daily rhythm of life is specific to species and their individual populations, individual developmental stages and fish in different biological states. It is developed as an adaptation to obtain food and defense against enemies. Fish are connected with their food objects and enemies by a complex system of relationships, one of the reflections of which is the daily rhythm of life. The diurnal rhythm varies depending on the environment.

A distinction is made between solar circadian rhythms (24 hours) and lunar, or tidal, circadian rhythms (24.8 hours). Daily rhythms characterizing the lifestyle of inhabitants are among the most clearly expressed biological rhythms. They are composed of the organism's reactions to diurnal changes in external conditions and spontaneous physiological rhythms. Under the influence of constant conditions, spontaneous circadian rhythms can change their period, turning into circadian or circadian rhythms.