Aims: Why are some healthy male shift workers (SWers) overweight [body mass index (BMI) >25 and <30] if not obese (BMI >30)? Seven risk factors potentially causing overweight and obesity were evaluated, namely (1) age, (2) physical/sports activity, (3) length of exposure to shift work (SW), (4) speed of shift rotation, (5) tolerance to SW, (6) internal desynchronization of circadian rhythms and (8) night eating (nocturnal nibbling). “New” as well as “old” data, acquired from longitudinal and individual time series of 5–56 days recording span, were reanalyzed. The data were analyzed from a set of field studies of 67 SWers and 53 non-shift workers (non-SWers). To estimate the respective weight of these factors, a multiple regression analysis (MRA) was used among other statistical tools. A similar age-related increase in BMI was validated (with p < 0.001) in both SWers and non-SWers. However, in SWers, desynchronization of rhythms increases the effect of age on BMI. Length of exposure to SW, tolerance to SW and speed of rotation do not seem to play a role as risk factors. Major effects are likely to relate to a sedentary lifestyle (lack of regular physical or sport activities) (MRA with p < 0.01), as well as, presumably, to a nocturnal nibbling of carbohydrates, which mimics the night eating syndrome.