Deep sleep is when growth hormone actually gets released — the stuff responsible for rebuilding muscle, repairing tissue, and regulating fat storage. Most veterans I talk to are running on 5-6 hours and calling it normal because that's what years of shift work and deployments trained them to accept.
Here's the problem: cut sleep short, and you don't just feel tired. You blunt the exact hormone response your body needs to recover from training at all. You can eat perfectly and train hard and still stall out — because the repair window never opened.