If you're clearheaded, you're wide awake, focused, and thinking sensibly. A clearheaded student will probably do better on a calculus test than her sleepy, confused classmate.
Complex thought and analysis ideally requires a clearheaded thinker — and you can also describe the thinking and decisions themselves as clearheaded. A clearheaded debate between political candidates is much more useful and interesting than a muddled, angry, and confusing argument. The word clearheaded, sometimes spelled with a hyphen, clear-headed, originated in the early 18th century.