High sleep quality has been associated with beneficial outcomes across the life span. Intensive longitudinal studies suggest that these beneficial effects can also be observed on a day-to-day level. However, the dynamic interplay between subjective sleep quality and affective well-being in children's daily life has only rarely been investigated. The aims of the present work were (a) to replicate findings from a prior ambulatory assessment study in this area (Könen et al., 2016), (b) to explore the effect of subjective sleep quality on well-being throughout the day, and (c) to examine the reciprocal relation between subjective sleep quality and well-being in more detail. Data from two ambulatory assessment studies with children between 8 and 11 years (N = 108/84, with assessments over 28/21 consecutive days) consistently showed that positive affect was higher and negative affect was lower after nights with better sleep quality, and that the effects of subjective sleep quality were stronger on well-being assessed in the morning compared with later in the day. Results from dynamic structural equation models revealed reciprocal effects of subjective sleep quality and positive affect. Negative affect was not consistently related to worse subsequent sleep quality after controlling for positive affect and prior night's sleep quality. Results suggest a close relation of sleep quality and positive affect, which strengthens the idea behind interventions targeting both, children's sleep and well-being. Differences between children in the dynamic interplay between sleep and affect may be important predictors of long-term outcomes. (PsycInfo Database Record (c) 2021 APA, all rights reserved).