Timed light exposure can be effective to help people match their circadian rhythms with the expected cycle at their destination; it requires strict adherence to timing. Light therapy is a popular method used by professional athletes to reduce jet lag. Special glasses, usually battery-driven, provide light to the eyes, thus inhibiting the production of melatonin in the brain. Timed correctly, the light may contribute to an advance or delay of the circadian phase to that which will be needed at the destination. The glasses may be used on the plane or even before users leave their departure city.