A sodium vapour lamp is a gas discharge lamp that uses sodium in an excited state to produce light. There are two varieties of such lamps: low pressure and high pressure. Because sodium vapour lamps cause less light pollution than mercury-vapor lamps, many cities that have large astronomical observatories employ them. At the end of life, high-pressure sodium lamps exhibit a phenomenon known as cycling, which is caused by a loss of sodium in the arc. As a result these lamps can be started at a relatively low voltage, but as they heat up during operation, the internal gas pressure within the arc tube rises, and more and more voltage is required to maintain the arc discharge.