Four Day Work Week

I love having four day work weeks but doesn't that just mean we have to do the same amount of work in less time?  Ok . . . I did get somewhat of a break since I didn't have to lead and complete the documentation for my Monday groups.  But I ended up staying at work two hours late yesterday finishing a lot of the work I normally get done on Mondays.  Tonight I'm just exhausted.  Anyone else notice this one tiny problem with four day work weeks?