This study examined the long-term associations between reports of sleep duration and diurnal cortisol secretion in older adulthood. It was hypothesized that longer sleep would protect older adults against increases in diurnal cortisol secretion over time. We tested this hypothesis using three waves of data from a 4-year longitudinal study involving 157 older adults. Results from growth curve and cross-lagged panel analyses demonstrated that levels and increases in sleep duration buffered long-term elevations of diurnal cortisol secretion. Reversed analyses indicated that diurnal cortisol secretion did not predict changes in sleep duration over time. These results were independent from sociodemographic characteristics (i.e., age, sex, partnership status, and education) and health-related variables (i.e., chronic illness, medication usage, body mass index, and smoking). They suggest that long sleep exerts restorative functions and protects older adults from exhibiting increases in diurnal cortisol secretion over time.
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