We are always talking about 4-day work week or stuff like in this article. However, I am much more interested in the question how much hours one could chip off

We are always talking about 4-day work week or stuff like in this article. However, I am much more interested in the question how much hours one could chip off of the usual 8 hour (office) work day and still be basically 100% productive. My guess is that around 6-7 hours should probably be AS productive as 8 hour-days. But, of course, this is dependend on a lot of factors (e.g. do you still have enough opportunities to communicate with colleagues etc.) which is why I would really like to see more studies about it. Personally, I have just reduced my hours from 40 to about 35 and I love it. It's not just the extra hour that I have in the evening but also that I am way less tired after work (thus can enjoy the rest of the day more). Also, this last hour (to me) always felt like "I have to do it although it ain't productive at all". But that's me. Just one anecdata. I would like more.