Despite growing evidence that there is substantial nightly, intra-individual variability in sleep duration and fragmentation, few studies have investigated the correlates of such variability. The current study examined whether intra-individual variability in sleep parameters was associated with psychosocial and physiological indices of stress, especially among those high in negative affect. Participants were 184 adults aged 46-78 (53% men and 41% Black) in the Pittsburgh SleepSCORE study. Wrist actigraphy was used to estimate sleep duration and fragmentation for nine nights, and overnight samples of urinary norepinephrine were collected for two nights. Stressful life events, depression, and anxiety were also reported. Intra-individual differences exceeded between-person differences in actigraphy-measured sleep duration and fragmentation. Stressful life events were associated with increased nightly variability in duration and fragmentation (ps<.05). Negative affect moderated associations between norepinephrine and variability in sleep, such that the greatest variability in actigraphy measures was among those with both high norepinephrine levels and high negative affect (ps<.05). These data suggest that both psychosocial and physiological stress are related to increased nightly variability in individuals' sleep duration and fragmentation, particularly among those reporting negative emotions. These results may have implications for both sleep and health research.