• Depression;
  • total plasma tryptophan;
  • diurnal variation;
  • sleep reversal;
  • fasting

ABSTRACT– Total plasma tryptophan was analysed during a 24-h period in 27 control persons and 42 patients with a diagnosis of endogenous depression. Total plasma tryptophan was unchanged in endogenously depressed patients compared with controls. The effect of one night's sleep reversal was investigated in control persons. During the following day, when the persons were asleep, plasma tryptophan concentration fell below control values. The normal diurnal variation with low values in the night and high values in the daytime disappeared when the persons were kept fasting.