Most people will have some warning that their blood glucose levels are too low, which gives them time to correct them. Symptoms usually occur when blood sugar levels fall below four millimoles (mmol) per litre. Typical early warning signs are feeling hungry, trembling or shakiness, and sweating. In more severe cases, you may also feel confused and have difficulty concentrating. In very severe cases, a person experiencing hypoglycaemia can lose consciousness. It's also possible for hypoglycaemia to occur during sleep, which can cause excess sweating, disturbed sleep, and feeling tired and confused upon waking.