When you watch a child put so much focus and energy into something they're excited about it's fun to watch.  Then you ask the child to do something that they may not be interested in...and the excuses come out.  Especially "I'm tired".  If we expand this out to a bigger picture into our lives, do we exhibit the same behavior?  Are we "tired" because we lack motivation toward what we're doing or are we truly tired.  I believe most of the time it's the former not the latter.  Time to do some introspect on your life!