The temporal organization of eukaryotic energy metabolism is considered to be an evolutionary adaptation to a cyclic energy supply from the environment. Circadian rhymthicity in energy transduction seems to be essential for activity regulation between subsystems of energy metabolism for the benefit of the entire system. It seems probable that in the course of evolution, the circadian system became a genetic program that provides the temporal frame for physiological and behavioural patterns that is necessary for the proper functioning of organisms and populations. It is very likely that circadian changes in energy metabolism will be reflected in corresponding oscillations in the state of cell membranes which carry the various photoreceptors of the cell. The interaction of photoreceptors and membranes could therefore be controlled by energy metabolism. Photostimulation of the photoreceptors could modulate rhymthic energy transduction while the energy dependent state of the membranes could determine the sensitivity of membrane-bound receptors and transport systems. The membranes very probably are also the transducers for temperature signals. Circadian oscillations in membrane functions are therefore considered to be the basis for photo- and thermoperiodic phenomena displaying rhythmic changes in sensitivity. The importance of specific rhymthic interactions between subsystems of the cell is demonstrated by imposing photo- and thermoperiodic cycles with artificial phase relationships which do not exist in nature and therefore are barely tolerated by the system. Changes in phase-relationships between rhythmic metabolic pathways i.e. internal desynchronisation may be lethal for the organism.