Objectives: Bedtime routines are a highly recurrent family activity with important health, social and behavioural implications. This study examined perceived barriers to, and facilitators of, formulating, establishing, and maintaining optimal bedtime routines in families with young children. Design: Participants completed a semi-structured interview based on the Theoretical Domains Framework (TDF). Analysis followed a deductive approach. Participants: A total of 32 parents participated in the study. Most participants (N = 30) were females, were white (N = 25) and stay at home parents (N = 12). Results: Key barriers included lack of appropriate knowledge and sources of information, problematic skills development, social influences, cognitive overload, and lack of motivation for change. Facilitators included social role, access to resources, positive intentions, beliefs about consequences and reinforcement. In particular, optimal bedtime routines were less likely to be enacted when parents were tired/fatigued and there was a strong effect of habit, with suboptimal routines maintained over time due to past experiences and a lack of awareness about the importance of a good bedtime routine. Conclusions: Several theory-based, and potentially modifiable, determinants of optimal bedtime routines were identified in this study, providing important information for future interventions. Several of the key determinants identified were transient (tiredness) and/or non-conscious (habit), suggesting that future interventions may need to be deployed in real time, and should extend beyond conventional techniques.