From Don Whitney's Spiritual Disciplines for the Christian Life:
To use time wisely "because the days are evil" is a curious phrase embedded in the inspired language of the Apostle Paul in Ephesians 5:15-16: "Therefore be careful how you walk, not as unwise men, but as wise, making the most of your time, because the days are evil" (NASB). Paul may have exhorted the Christians at Ephesus to make the most of their time because he and/or the Ephesians were experiencing persecution or opposition (such as in Acts 19:23-20:1). In any event, we need to use every moment with wisdom "because the days are evil" still.
Even without the kind of persecution or opposition known by the Christians of Paul's day, the world we live in is not conducive to using time wisely, especially for purposes of spirituality and Godliness. In fact, our days are days of active evil. There are great thieves of time that are minions of the world, the flesh, and the Devil. They may range in form from high-tech, socially acceptable preoccupations to simple, idle talk or ungoverned thoughts. But the natural course of our minds, our bodies, our world, and our days leads us toward evil, not toward Christlikeness.
"Thoughts must be disciplined, otherwise, like water, they tend to flow downhill or stand stagnant. That's why in Colossians 3:2 we're commanded, "Set your mind on the things above." Without this conscious, active, disciplined setting of the direction of our thoughts, they will be unproductive at best, evil at worst. Our bodies are inclined to ease, pleasure, gluttony, and sloth. Unless we practice self-control, our bodies will tend to serve evil more than God. We must carefully discipline ourselves in how we "walk" in this world, else we will conform more to its ways rather than to the ways of Christ. Finally, our days are days of active evil because every temptation and evil force are active in them. The use of time is important because time is the stuff of which days are made. If we do not discipline our use of time for the purpose of Godliness in these evil days, these evil days will keep us from becoming Godly."
Tim Challies offers a book review of Spiritual Disciplines here. I heartily recommend reading it.