For decades, psychologists around the world have studied why people exercise — and why they don’t — and there’s a growing body of work dedicated to helping you get up off the couch.

Self-control comes in limited quantities, must be replenishedCENTER FOR THE ADVANCEMENT OF HEALTH -- Self-control, whether used to pass up the office cookie plate or to struggle against temptations like alcohol and tobacco, operates like a renewable energy source rather than a learned skill or an analytical thought process, according to new research.