<![CDATA[Background: Sleep duration, chrononutrition, and meal timing are emerging determinants of metabolic health. Circadian misalignment, caused by sleep restriction or irregular eating patterns, may adversely affect body composition by promoting adiposity and reducing muscle preservation through neuroendocrine and molecular dysfunctions.Objectives: This review synthesized research findings from the past decade (2016–2026) to evaluate how sleep duration and breakfast energy intake influence body composition parameters, specifically fat-free mass (FFM), basal metabolic rate (BMR), and cellular phase angle (PhA).Methods: Literature search was conducted in PubMed and Google Scholar. From 306 identified papers, 12 original research studies involving diverse populations (including children, adolescents, and adults across multiple continents) were included. The review analyzed observational and experimental data focusing on sleep patterns, meal frequency, and timing as primary exposures, with body composition metrics as the main outcomes.Results: Experimental sleep restriction (4 hours/night) significantly increased caloric intake (P = 0.015) and visceral fat (P = 0.042). Optimal sleep (6–7 hours) was associated with more favorable body composition than extreme durations (P < 0.05). Regarding chrononutrition, habitual breakfast skipping was positively correlated with lower appendicular skeletal muscle mass index (β = 0.087, P = 0.031). In large-scale observational data, night eating increased metabolic syndrome prevalence by 48% (OR: 1.48; 95% CI: 1.15–1.90), while morning eating was protective. Each one-hour delay in eating timing was associated with a 1.25-unit increase in body fat percentage (P = 0.010). Phase angle, reflecting cellular integrity, improved with regular breakfast consumption (r = 0.15) and physical activity. Notably, physical activity often moderated these relationships, acting as a stronger predictor of FFM than sleep in specific demographics.Conclusion: Sufficient sleep and structured, early-timed meal patterns, particularly regular breakfast consumption, support lean mass preservation and reduced adiposity. Maintaining consistent sleep-wake cycles and physical activity are essential synergistic strategies to mitigate the metabolic risks associated with circadian misalignment.Keywords: Body Composition; Sleep; Breakfast; Circadian Rhythm; Basal Metabolis]]>
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