We've been looking at our thinking minds and have become very aware of our inherent distractibility. As we practice and build our powers of concentration, our minds begin to truly rest. Often, at first, chill and alert seem to be incompatible – with our alertness being driven by stress, and our chill/calm having a dull or checked-out quality.
The key in mindfulness practice is to find the subtle balance of having both! You'll find that this quality is present when you're in a (low-stress and high-performance) state of flow.