Sleep is where the adaptation happens. The training session is the stimulus — but muscle protein synthesis, hormonal recovery, and metabolic repair all occur during sleep. Compromise the sleep, and you compromise the return on every hour you spend training.
The hormonal mechanism
Sleep restriction (less than 7 hours) consistently elevates cortisol, reduces growth hormone secretion, and impairs insulin sensitivity. Each of these effects works directly against body composition goals.
Elevated cortisol promotes muscle protein breakdown and fat storage — particularly visceral fat. Growth hormone, which drives the majority of overnight muscle repair, is secreted primarily in the first few hours of deep sleep. Truncate that window, and you lose a significant portion of the anabolic response to your training.
In a landmark study from the University of Chicago, participants on a calorie-restricted diet lost the same total weight whether they slept 5.5 or 8.5 hours. But the composition of that weight loss was dramatically different: the 8.5-hour group lost 50% of their weight as fat. The 5.5-hour group lost only 25% as fat — the rest was lean mass.
You can be in a perfect caloric deficit, training consistently, and still preferentially lose muscle if your sleep is inadequate.
Appetite and intake
Poor sleep also affects body composition through appetite. Sleep deprivation increases ghrelin (hunger hormone) and decreases leptin (satiety hormone), consistently producing a net increase in caloric intake of 200–500 kcal per day in controlled studies. The food choices made in sleep-deprived states also skew heavily toward high-calorie, high-carbohydrate options.
What we assess and address
At KRAFT, sleep quality is part of our initial assessment — not an afterthought. We review sleep duration, consistency, architecture (through wearable data where available), and the lifestyle factors driving poor quality.
For most clients, the interventions that make the biggest difference are simple: consistent sleep and wake times, controlled light exposure in the evening, and alcohol reduction. More complex cases — sleep apnoea, chronic stress, shift patterns — require a structured protocol and, where appropriate, referral.