How short sleep changes body composition during a diet
When calories are restricted, the body draws on both fat and lean tissue. Research on sleep restriction during weight loss has found that, at the same calorie deficit, inadequate sleep tends to shift a larger share of the weight lost toward lean mass rather than fat. A frequently cited 2010 trial by Nedeltcheva and colleagues reported that participants on restricted sleep lost more of their weight as fat-free mass and less as fat compared with adequate sleep.
In other words, the deficit can be identical while the outcome is worse, because the partitioning of the loss changes unfavorably.
The hormonal and behavioral pathways
Sleep loss is associated with shifts in appetite-regulating hormones, including higher ghrelin and lower leptin in some studies, which can increase hunger and cravings. It also tends to raise cortisol and impair insulin sensitivity, both of which can influence how the body manages energy and stores fat.
Behaviorally, fatigue reduces training quality, recovery, and adherence. The practical implication is that prioritizing sufficient sleep supports the goal of preserving muscle while losing fat, though individual responses vary and findings come from controlled studies rather than guarantees.