Biorhythms in toxicology and pharmacology. I. Generalities, ultradian and circadian biorhythms.

According to their periods, rhythms can be classified in ultradian (period inferior to one day), circadian (period equal to one day) and infradian (period superior to one day). Different biorhythms of endogenous origin are modified mostly relatively to their amplitude by definite doses of the effects of pharmacological or toxic agents. Circadian variations of the effects of drugs, of the toxicities of gases as well as circadian modifications of the reactions to different stresses and agressions have been demonstrated amply in animals (mostly nocturnal rodents) and sometimes in man. In a general way reactions are more rapid and more marked in the active period where enzymatic reactions show a peak activity corresponding to a maximal energy output than during the rest and sleep periods.