Summarize:

Keep track of how long you can ordinarily work before getting tired. Once you have this benchmark, try working more than this on one day, followed by a day in which your work load is lighter. Periodically pushing yourself like this can help your build up to being able to work effectively for longer periods. Another technique to building up to longer periods of work is to push yourself a little further at times. When you feel like you don't want to work any longer, or are looking for ways to procrastinate, don't stop immediately. Instead, commit to working a little longer (20 minutes, for example), and then stopping. Getting enough rest is important to maintaining a good work ethic. If you are truly tired (and not looking to procrastinate), it is a sign that you should take a break. Whatever your work schedule is like, make sure you factor in enough time for sleep and relaxation into your day. There is some variation from person to person, but generally adults need 7.5 to 8.5 hours of sleep per day. Work can be very important, but it is also important not to neglect other parts of your life. Everyone needs mental breaks, to have time to do things they enjoy, and to take care of non-work priorities. Maintaining a life beyond work is actually part of developing a good work ethic, so make sure you allow yourself this. When you are off of work for the day, disconnect completely. Try not to think about pending projects, check your work email, or talk too much about work with family and friends.
Build up to longer periods of work. Give yourself time to rest. Maintain a good work-life balance.