One moment, your leg feels completely normal. The next, a muscle suddenly tightens into a painful knot. You may freeze in place, waiting for the contraction to release. Sometimes it lasts only seconds; other times, the pain continues for several minutes and leaves the affected muscle sore afterward. Leg cramps are common, particularly in the calves, but they can also affect the thighs, feet, and other muscles.

A muscle cramp is a sudden, involuntary contraction in which a muscle tightens and temporarily refuses to relax. Most cramps are harmless, but there isn’t one universal explanation for why they happen. Muscle fatigue after strenuous or unfamiliar exercise is one possibility, while heavy sweating and dehydration can contribute in certain situations. However, many people experience nighttime cramps without being dehydrated at all. And when cramps keep returning, the cause may involve something you wouldn’t immediately associate with the muscle itself.