Obstructive sleep apnoea (OSA) is linked to increased cardiovascular risk. This risk can be reduced by nasal continuous positive airway pressure (nCPAP) treatment. As OSA is associated with an increase of several vasoconstrictive factors, we investigated whether nCPAP influences the digital volume pulse wave. We performed digital photoplethysmography during sleep at night in 94 consecutive patients who underwent polysomnography and 29 patients treated with nCPAP. Digital volume pulse waves were obtained independently of an investigator and were quantified using an algorithm for continuous automated analysis. In patients with OSA and an apnoea/hypopnoea index (AHI) of >10 events · h(-1), a significant vasoconstriction was observed during the night (p<0.0001 by Friedman's test). A significant positive correlation existed between vasoconstriction and AHI (Spearman correlation, r = 0.27; p<0.01; n = 94) and the arousal index (Spearman correlation, r = 0.21; p < 0.05; n = 94). After 6 months of nCPAP treatment, the AHI was significantly reduced from 27 ± 3 events · h(-1) to 4 ± 2 events · h(-1) (each n = 29; p<0.001) and vasoconstriction during the night was significantly reduced from 10 ± 3% to 3 ± 1% (p<0.01). We show changes in the reflective index during the night consistent with vasoconstriction in patients with OSA, which are significantly reduced after 6 months of nCPAP treatment.