Some days, you get up energized and hit the road or the gym, and you feel like a million bucks. And some days, it’s just a grind. Why? It could be that on those down days, your heart rate variability (HRV) is low and your autonomic nervous system is out of balance. Training hard on low-HRV days is just not as effective; you might want to consider taking it down a notch or two–or even skipping training that day. You can use the SweetBeat™ iPhone app to monitor your HRV first thing in the morning to assure your workouts are more effective.