This is pretty clear – you sleep less, you feel more tired and you don’t work out, Malkoff-Cohen says. The more sleep-deprived you are, the more lethargic you feel, and the less you will exercise. Going back to a sedentary lifestyle will inevitably result in weight gain, especially if you’re eating more. Studies have shown clear evidence that low levels of physical activity are associated with an increased risk of weight gain and obesity.