Biological and Habitual Factors That Influence Sleep and Circadian Rhythm: A Narrative Review.

Sleep loss and circadian misalignment can adversely affect health. As such, a comprehensive understanding of what factors influence sleep loss and circadian misalignment benefits medical and scientific progress. While the interaction between light exposure and circadian entrainment and their relationship to circadian alignment is long-established, other factors that influence central or peripheral clocks (bodily oscillators) also contribute to circadian alignment and the potential for sleep loss. In this review, we will highlight the core factors that influence vulnerability or resilience to sleep loss and circadian misalignment, including biologic factors, such as gene haplotype, chronotype, and the gut microbiome. Furthermore, we will consider how habitual and largely modifiable variables, such as timing of meals and physical activity, can be manipulated to affect circadian and sleep cycles. Gaps in current scientific research on the effects of circadian misalignment and sleep loss will also be identified. This review will thus help inform future research and enhance the understanding of how healthy sleep and balanced circadian rhythm can improve quality of life.