Ecological momentary assessment (EMA) can capture how sleep, stress, and well-being are related within individuals. However, the use of EMA involves participant burden, which may be a major barrier when studying at-risk populations like frontline workers. To guide future research interested in using EMA, this study examined variance components in sleep, stress, and well-being variables collected from health care workers. Two samples of hospital nurses (60 inpatient, 84 outpatient) responded to 2-week smartphone-based EMA. Adherence to the EMA protocol was good in both samples. Results from intraclass correlations showed more momentary variability in stressors and uplifts, more daily variability in sleep, fatigue, and physical symptoms, and more between-person variability in affect, rumination, and work quality. Across the variables, however, there was substantial within-person variability. Variance components were relatively consistent between workdays and non-workdays and between week 1 and week 2. Some nuanced between-sample differences were noted.