No study to date has investigated whether the activity of circulating dipeptidyl peptidase 4 (DPP-4) is affected by sleep loss. DPP-4 is an enzyme that catalyzes a variety of important physiological processes in humans by cleavage of, e.g., the incretin hormones glucagon-like peptide 1 (GLP-1) and gastric inhibitory polypeptide (1). Both chronic sleep loss and an increased activity of DPP-4 have been implicated in the development of several diseases, including type 2 diabetes (1-4). Twenty-five

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... normal-weight healthy adults (aged 18-28 years; 13 women, using oral monophasic contraceptives) participated in two in-laboratory experimental conditions separated by about 1 week: one night of regular sleep (scheduled 2230-0630) and one night of total sleep loss, in a counterbalanced order. In the morning (;0730), fasting blood samples were taken to measure the enzymatic activity of DPP-4, as previously described (3). The effects of sleep loss on DPP-4 Figure 1-Measured enzymatic activity of circulating DPP-4 after a full night of sleep and after a night of total sleep loss in women and men. Left: DPP-4 activity following sleep (black bars) and total sleep deprivation (TSD) (red bars). Numbers are derived from the estimated means for the main effect of experimental condition (sleep/TSD) in the linear mixed model (N 5 25). Middle: DPP-4 activity in women (N 5 13) and men (N 5 12). Numbers are derived from the estimated means for the main effect of sex in the linear mixed model. Right: Mean DPP-4 activity following sleep as well as TSD, split by sex. Wilcoxon signed rank test was used to test for differences in DPP-4 activity between the experimental conditions (sleep/TSD) in women (N 5 11) and men (N 5 12), respectively. The linear mixed model showed an interaction between experimental condition and sex (P 5 0.029). The direction of the interaction is demonstrated by this graph. One of the data points for two of the participating women are missing due to technical and blood drawing failure. Therefore, these two participants were not part of the Wilcoxon signed rank tests (but included in the linear mixed model), nor were they included in the means used to generate the right panel. Error bars represent SEM. AU, arbitrary units; n.s., not significant. *P , 0.05; ***P , 0.001.