Sleep: The Overlooked Pillar of Muscle Hypertrophy—A Critical Review of Mechanistic and Observational Evidence

Muscular strength and skeletal muscle mass are central to human performance and functional capacity across the lifespan. Beyond sport, low strength and poor muscle quality are associated with adverse health outcomes and reduced quality of life. Hypertrophy research has traditionally emphasized training-derived stimuli such as mechanical tension, muscle damage, and metabolic stress; however, adaptation is also shaped by recovery behaviors, with sleep increasingly recognized as a modifiable determinant of training readiness. This review synthesizes evidence on sleep in relation to hypertrophy- and strength-related adaptations. We briefly summarize established drivers of muscle growth, then evaluate mechanistic pathways linking insufficient sleep to altered endocrine signaling, muscle protein turnover, and neuromuscular performance. We also review observational and experimental studies examining sleep duration and sleep restriction in relation to lean mass, strength, and training outcomes, noting that short sleep—and, in some cohorts, long sleep—has been associated with poorer muscular health. Finally, we distinguish the evidentiary strength of observational, mechanistic, and longitudinal training studies; propose a conceptual model in which sleep can constrain or support resistance-training adaptations; and identify priorities for studies that manipulate sleep in resistance-trained participants.