While training for the first Ironman, I changed jobs to one that required a commute and a fixed "out the door" time to beat traffic. After starting this, every Friday was cold-day. I'd get so rundown from a lack of sleep, only 8 hours/night, that I'd get a cold every Friday. After knocking off for 10+ on Friday night it'd be gone Saturday.
Sleep is also prominent in modern training programs. The old school train-till-you-die method of unfocused bulk mileage and little rest is gone. Nowadays endurance athletes have narrowly focused training workouts (speed work, intervals, weights, long & easy, etc) with a balance of days off and entire weeks of reduced training volume for recovery where the emphasis is put on rest so the body can rebuild and prepare for the next training cycle.