Nocturnal insulin and C-peptide rhythms in normal subjects.

Immunoreactive insulin and C-peptide reactive plasma levels in venous blood were studied between 0100 and 0820 h in 10 healthy volunteers to assess nocturnal endogenous insulin secretion and its peripheral extraction. Insulin secretion appeared significantly reduced after 0600 h. No correlations were observed between counterregulatory hormones and insulin secretion during this period. Plasma glucose remained stable throughout the study. An increased peripheral sensitivity to insulin after 0600 h is suggested to explain reduced secretion of insulin.