Can you enumerate this further? Are you getting measurable and actionable feedback from a superior, or do you just feel like you should have accomplished something with the additional half of a day?
I'm going to reaching a little as I don't know you're specifics, but I've found myself in this situation before and as a manager I've had to help my reports with the same issues. Typically when I hear this from my reports, their behavior falls into one of three categories:
1. They're still working on things, but they don't contribute to what they see as their primary goal. I.e. I ask them to rework something, they have to dig through documentation/emails/etc to answer someone's question, or they have some HR function they have to complete. In the first example it's often times expressed as "This took me all month!"
My suggestion in these situations is to realize you're being "alternatively productive." My ask that someone rework something or performing duties for another team is still part of being productive. Not all managers manage that way so if you're running into trouble here, I'd bring it up and discuss it further.
2. They're not being productive and legitimately are just taking some downtime and feel bad about it.
Assuming I've gotten 3.5 days of good productivity out of someone, I ask them to embrace their downtime. Take a walk, chat with folks, read some blogs, or just simply go home early. I'd rather someone come back refreshed the next day than waste four hours feeling bad at their desks and contribute to a future episode of burn out.
Some managers are driven to make sure butts are in seat coding 40+ hours per week. Thankfully my company doesn't work that way.
3. There is a performance issue. Sometimes that's on me (poor direction giving, unclear instruction or goals, being too busy doing my other duties, etc) and sometimes it's on them. For example their 3.5 days of productivity and I need to help mentor or drive them. That's where immediate, specific, actionable, and measurable feedback comes into play.
That's something a lot of managers are bad at. I'm aware that feedback itself has a quality component that should be met and still do a bad job at times.
I hope something here is helpful for your situation.