Agree with woofgang - if I'm tired a have a wee nap even during the day. I read, iron and do bits and pieces of housework at night if I'm not tired. Not easy if you've to work the next day though.
Try this when you feel too weary to keep going :
sit at a table and rest your head on your arms. You won't sleep for as long as you would if you got comfortable in an easy-chair, but it will take the edge off your weariness if you just doze for 15 - 20 minutes