Loneliness, positive social interactions, and diurnal cortisol among mid-to-later life adults in everyday life.

Loneliness, the subjective feeling of social isolation, is a major public health concern linked to poor health, including stress-related diseases such as cardiovascular disease. However, less is known about how loneliness relates to biological functioning in everyday life and whether age and social interactions shape these associations. We investigated whether loneliness self-reported in everyday life related to the dynamic range of diurnal cortisol (CDR) compared to other indicators of diurnal cortisol (including the cortisol awakening response [CAR], diurnal cortisol slope [DCS], and area under the curve [AUC]). We also tested whether associations varied by age and frequency of positive social interactions. Data come from a national daily assessment study with participants ranging between 34 and 83 years old who completed 8 days of daily diary assessments (N = 626) and 4 days of salivary cortisol collection. Linear regression analyses showed that people with higher average daily loneliness had more compressed levels of CDR (b = -2.66, SE = 0.74, p < .001), an association that did not differ by age (b = -0.04, SE = 0.06, p = .46). The association between loneliness and CDR varied as a function of the number of days with positive social interactions (b = 0.90, SE = 0.18, p = 0.02), such that the association was smaller among people who reported more positive social interactions. No associations were found for CAR, DCS, or AUC. Findings suggest that greater loneliness in daily life is associated with reduced HPA-axis flexibility, particularly among individuals with fewer positive social interactions.