The circadian system is primarily entrained by the ambient light environment and is fundamentally linked to metabolism. Mounting evidence suggests a causal relationship among night-time light exposure, shift work, and metabolic disease. Previous research has demonstrated altered metabolic phenotypes elicited by chronic (> 4 weeks) exposure to dim light at night (DLAN) (~5 lux). However, the metabolic effects of short term (< 2 weeks) exposure to dim light at night are unspecified. We hypothesized that metabolism is perturbed by just two weeks of dim light at night. Specifically, we predicted that mice exposed to dim light would gain more body mass, alter whole body metabolism, and display altered body temperature (T_b) and activity rhythms compared to mice maintained in dark nights. Our data largely support these predictions; DLAN mice gained significantly more weight, reduced whole body energy expenditure, increased carbohydrate over fat oxidation, and altered circadian temperature rhythms. Importantly, these alterations occurred despite similar activity locomotor levels (and rhythms) and total food intake between groups. Peripheral clocks are potently entrained by body temperature rhythms, and the deregulation of body temperature we observed may contribute to metabolic problems due to ‘internal desynchrony’ between the central circadian oscillator and temperature sensitive peripheral clocks. Endocrine changes will also be presented.  We conclude that even relatively short term exposure to low levels of nighttime light can influence metabolism to increase weight gain.