I'm 41 and I work out at 5:30am most days (have been on this schedule for a little over a year). I agree w/ the PPs who emphasized a warm up. My walk to boot camp takes about 15m, and I am usually loosened up when I get there, and then there is a short warm up there as well. On the days I run, I also walk for about 5-10 minutes, gradually increasing my pace. Also, if I stay up to late, I am always less coordinated and my workouts are harder!