You wake up with jaw pain, tightness in the face, a stiff neck and a headache before you've even had coffee. Most people normalise this and chalk it up to stress or poor sleep. But behind the teeth-grinding - known as bruxism - there's often something less obvious: a magnesium deficiency.

Magnesium is key to dozens of processes in the body, but most important is that it regulates the nervous system and keeps the balance between muscle tension and relaxation. "When levels are low, the body can get stuck in a constant state of tension," explains a pharmacist and nutritionist. With little magnesium, the jaw struggles to relax and the nervous system stays over-activated - the ideal ground for nighttime grinding.

"Bruxism isn't always just stress," experts stress. Many adults today have an over-activated nervous system and muscles that are almost permanently clenched. A magnesium deficiency is especially common in women, due to hormonal changes, contraceptive use, digestive problems or a mineral-poor diet.

Still, here comes an important caveat. Doctors warn that no supplement is a complete fix for bruxism - the scientific evidence that magnesium alone removes the problem is insufficient. Which means: if you grind your teeth regularly, the first step is a doctor, not the pharmacy. A supplement can help with the muscle tension, but it doesn't replace a diagnosis.

The good news is that magnesium doesn't have to come from a bottle. Leafy green vegetables, legumes, walnuts, seeds, wholegrains, pumpkin and sesame seeds, almonds and pure cocoa are natural sources. Dark chocolate with a high cocoa content is perhaps the most pleasant way to help yourself - advice few will turn down.