Sunlight is our main sleep synchronizer. Therefore exposure to ultra bright light is successfully used in sleep disorders that are caused or accompanied by a dysfunction of the circadian system. Phototherapy has concrete indication in cases of sleep phase delay, circadian deregulation, phase advance, free-running sleep, jet lag, shift work, some cases of insomnia in the elderly and in some cases of depression or illness Alzheimer. If applied in the morning after waking up, the light will lead to a phase advance, which means that one is going to fall asleep sooner. On the other hand, light applied at the end of the day will give rise to a phase delay.