Circadian rhythm of plasma tryptophan in clinically healthy subjects and patients with endogenous depression.

Variations of plasma tryptophan along the 24-h scale are studied in clinically healthy subjects and in patients with endogenous depression. In both groups a circadian rhythm in free and total tryptophan has been found. In healthy subjects plasma tryptophan levels were higher during the night in comparison with daytime hours; the opposite was demonstrated in depressed patients. The possible physiopathological mechanisms underlying the different behavior of plasma tryptophan in the 24-h span, in these two groups of subjects, are discussed.