I was on effexor for years and found I was tired *all* the time.  I would take 2 naps a day and seemed sapped of energy.  The thought of going out and doing things just seemed so much work.  Worked with my doc on several different ways to counter act it, but nothing really helped.  Taking Vit D helped a little (doc had me on up to 2000 units a day).  I finally got so sick of the side effects, I quit the effexor.  Dealing with a few effects from that, but it's not been as bad for me as some of the stories I have read from others that have gone off it (or tried). 

If you find yourself zoning out, take a 15 minute nap and try to get going again.  It's easy to nap longer, but then you mess up your regular sleep cycle which makes things worse during the day.