Most people experience leg cramps at some point in their life. From the occasional aching sensation to recurring pangs, people cope with leg cramps—also known as “charley horses” or muscle spasms—to varying degrees.

Cramps are involuntary contractions that occur in the muscles for anywhere from a few seconds to minutes on end (or even longer periods in some cases). Mild or severe, the onset of cramping is typically sudden; it may begin during intense exercise or at rest, in bed at night or after hours of being seated at your desk.

However, just because leg cramps can begin suddenly doesn’t mean they come out of nowhere. Muscle spasms in the calves, hamstrings, and thighs result from a number of common causes, many of which can be addressed simply by being aware of what might trigger them. These are some of the main causes of leg cramps: