A bit ADDish. But I know people who are worse than I am...
Still, having done a little research and gained some perspective on it. Its not really a disorder...except in rare and extreme cases (usually ADHD - i.e. hyperactive). Having a short attention span is likely the product of human evolution...some people are exceptional at concentration but consequently are less aware...people with a short attention span are more aware of changes to their surroundings. For primitive humans...that means that the short attention span people will be the first ones to notice a predator sneaking up on a group.
Also, isn't SAD something you can treat by trying to get as much sunlight as possible? I'm not sure if thats true...but I thought it had to do

with the sun (the depression becoming marked during the winter and low light levels).