Background Poor sleep and affect are common, co-occurring symptoms in postmenopausal women. However, the temporal associations between these symptoms – whether sleep disturbances trigger next day changes in affect and/or whether affect disrupts next night sleep – is unknown. Here we examined the day-to-day bidirectional association of sleep with positive and negative affect in midlife postmenopausal women. Methods Midlife postmenopausal women (N = 240, mean age 59.42 ± 4.11 years, 82.08% White) completed 72 h of sleep and affect monitoring. Sleep characteristics (wake after sleep onset, sleep efficiency, total sleep time) were assessed via wrist actigraphy. Positive affect (energetic, interested/involved) and negative affect (angry/irritated, nervous/anxious, stressed) were assessed via ecological momentary assessment. Multiple linear regression models were conducted for all analyses. Moderation of the associations between sleep and affect by vasomotor symptoms and depressive symptoms was also examined. Results Better sleep (i.e., lower wake after sleep onset, higher sleep efficiency, and moderate total sleep time) was associated with higher next day positive affect but not negative affect. Higher positive affect was associated with shorter next night total sleep time. There was little evidence of vasomotor symptoms and depressive symptoms moderating these associations. Conclusion Findings support day-to-day associations between sleep and positive affect rather than negative affect. More robust associations were observed in the direction of sleep to next day positive affect compared with the positive affect to next night sleep direction. These findings highlight the potential to enhance positive affect in midlife postmenopausal women through sleep improvement.