Sex differences in the relationship between self-selected sleep-wake timing and morning cardiovascular risk in midlife adults.

Adverse cardiovascular (CV) events occur most frequently in the morning. People who wake up later in the morning appear to have an increased risk of these events, but the mechanisms causing this effect are unknown. Vascular endothelial function (VEF) is a prognostic marker for these adverse events. Thus, we hypothesized that later wake times are associated with impaired morning VEF. Moreover, since sex differences exist in sleep timing and cardiovascular physiology, we also explored sex differences in any such relationship between wake times and morning VEF. Twenty-four healthy volunteers self-selected a sleep schedule of 8 h in bed for at least one week and were provided with an equivalent 8 h sleep opportunity in the laboratory. We measured dim-light melatonin onset (DLMO) as a circadian phase marker from salivary samples and quantified sleep using polysomnography. After the 8-h sleep opportunity, we measured VEF via flow-mediated dilation. We tested correlations between self-selected sleep-wake timing and VEF. Results: In the group as a whole, there was no significant relationship between waketimes and VEF (r = -0.36, p = 0.16). However, in females, later wake time was significantly associated with attenuated VEF (r = -0.76, p = 0.03), and this was the complete opposite in males (r = 0.69, p = 0.04). Furthermore, in females, later DLMO (r = -0.77, p = 0.025) was also associated with attenuated VEF, but this relationship was not present in males. These initial data suggest that the time of awakening is associated with VEF during the vulnerable morning hours, but this relationship is the opposite between males and females.