I think there are a few factors to this question. Some people can very well be very involved in their project and want to see a project come to fruition. Which can very well be a motivating factor that causes some to work more than 40 hours a week. Another factor could be through watching others and believing that the norm is to work more than 40 hours a week. There are times at work currently when I feel awkward leaving work when the rest of my team is hard at work so that may entice me to stay longer than needed that day. This does not demotivate me but I can surely see over time how it can have an affect on some people. Overall, overworking can stem from a variety of factors and it is difficult to truly know if it will be a demotivating factor.