You wake up in the middle of the night in agony. Your calf muscle seems to have a life of its own and is in spasm, causing an agonising pain down the back of your lower leg. You try to force your leg to relax without success, and you know you are in for a sleepless night. What you have is cramp, or charley horse as it is also known in North America. This type of cramp is very common, particularly in the late stages of pregnancy. It occurs more frequently as people get older or if they put on weight, but it can happen to anyone, either during the night or after exercise. The cause of this common affliction has often been ascribed to low salt levels in your diet, or more precisely the sodium that is in salt. Take some salt and ease the pain, so the story goes, but as I will explain there is a much simpler, salt-free solution.Cramp occurs most often in the calf muscle at the back of the lower leg, the hamstrings at the back of the thigh or the quadriceps at the front. Occasionally it can indicate something more serious, such as claudication, a condition where insufficient oxygen reaches the muscles, causing them to tighten when a person walks. Or in rare cases, cramps can be caused by very low levels of calcium due to a problem with the parathyroid gland in the neck. But as Raymond Playford, Professor of Medicine at Barts and the London School of Medicine and Dentistry, says cramp is not associated with needing more salt in your diet in the majority of cases.Shock treatment ,What causes this cramp is something of a mystery. More than a century ago, people noticed that the men who stoked fires on ships were often afflicted, and it even became known as “stoker’s cramp”. This led to the theory that a lack of salt was the cause. The idea was that the heat of the fire caused the men to sweat so much that they became short of sodium. So naturally the assumption was that eating more salt would prevent the cramps.A biological explanation for this is that the lack of salt and accompanying dehydration causes the spaces between the cells of the muscles to contract, which then increases pressure on the nerve terminals, leading to pain. The problem with this explanation is the lack of robust evidence for this. To be fair this is not an easy topic to study. Because cramps are involuntary you never quite know when they will happen, making them difficult to investigate. If you assembled people in a lab for observation you could wait a very long time before they got cramp.This leaves observations of real-life environments, like the finding that American footballers suffer from cramp more when the weather is hot, lending further weight to the idea that, as with the stokers, this is all down to sweat loss and a lack of salt. The problem with this theory is that athletes in cold climates get cramp too. And when sodium loss was measured in athletes taking part in an ultra-marathon in Cape Town, South Africa, the difference between those who experienced cramp and those who did not was too small to be of clinical significance.