Your host Frieder introduces the topic of light and its influence on human physiology. Properties of light, such as the visible wavelength spectrum of light as well as ultra-violett (UV) and infrared light, are described. Differences between natural daylight and electric artificial light are pointed out. Time-of-day dependency of the influence of light on physiology is highlighted with a special focus on the secretion of the "darkness hormone" melatonin. Practical recommendations on how 24h light exposure can support wakefulness, sleep and overall health are mentioned and will be further discussed in the upcoming episodes with future guests. Frieder also presents his recent study on how an indoor 24h light scheme mimicking the natural light/dark cycle more closely influences metabolic health and thermoregulation in older volunteers with pre-diabetes.