Interesting anecdote:
At Marine Corps boot camp, and subsequently combat training, not enough sleep and plenty of exercise is the norm. I averaged somewhere around 5 hours a night, but never more than 7, and many times somewhere between 3 and 0. There wasn't a lot of pure exercise, but constant activity based exercise, and a lot of stress based exercise. I was mid 20s when I went through it, and had a couple of guys with me who had to get age waivers (30+). We all agreed it was the most alert we'd ever felt, and that when we slept, we slept hard and felt extremely well rested, even when sleeping on hard ground.
Since exiting I spent many years feeling lethargic and burnt out from work, until I started training for a marathon. Now I have that same feeling, that it doesn't matter how much I sleep, and my body just feels more alive. Exercise, especially aerobic and cardio vascular, does wonders for your health, if you have time to do it right.