Overexposure to positive ions lowers the blood and oxygen flow to the brain, which can result in inconsistent sleeping patterns. A Himalayan salt lamp's negative ions are supposed to reverse this effect, making them a popular sleep aid. These lamps are also known to improve mood and relieve stress. Salt itself is not toxic, but the glass container of a salt lamp can be if it breaks. As with all lamps, there is some evidence that salt lamps help people sleep better. But because high-quality sleep is important for health, any device used to assist with sleep needs to be evaluated by your doctor before you use it. For example, lamps may not be suitable for people who suffer from insomnia or other disorders of sleep regulation. Lamps contain metal wires that heat up when electricity is passed through them. This wire heating causes electrons to move between atoms in the metal, leaving behind negative ions. Ions are the only way light can escape a lamp; without them, the inside of the lamp would be completely opaque. People have used salt as a light source since the 16th century. The first electric lights were similar to modern halogen lamps except that they used carbonized bamboo instead of filaments. These lamps required constant maintenance to keep them working properly.