Millions of people worldwide regularly work during the evening or at night. These shift workers run an increased risk to develop metabolic diseases, including obesity and diabetes type 2. It is thought that shift work conditions disturb the well-regulated biological clock within the body, which may lead to disease. The biological clock, which is located in a central area in the brain, regulates many rhythmic behaviors and functions in collaboration with molecular rhythms generated in each cell in the body. The first part of this thesis aimed to study in rats how the timing of food intake, often changed by shift work, affects these rhythms at multiple locations in the body. It was found that rhythms of clock genes and neuropeptides in the hypothalamus of the brain, and clock and metabolic genes in the liver and the muscle respond differently to the abnormal timing of food availability. This desynchronization of organ responses may play a role in the reduced metabolic health. The second part of this thesis, contains a series of rat studies investigating how another feature of shift work, light exposure at night, affects glucose metabolism. A single exposure of light immediately caused glucose intolerance, and this effect was dependent on the time-of-day, intensity, color and duration of the light exposure. In addition, changes in the liver transcriptome were found after one hour of light exposure. The studies in this thesis contribute to the understanding of the underlying pathology on how shift work affects health.