Maybe humans need a little bit of everything? Short working hours interspersed with a few longer days, focusing on some days, and intense multitasking and coordination on others?

So if you're life currently involves only long days filled with multitasking (probably very common in some walks of life) the solution involves shorter days and more focused time, whereas for others it might be the other way around.

Whatever the truth is, I highly doubt humans are optimised specifically for exactly eight (or six) hours of focused work every day. I suspect we are helped by some variation, including the occasional 14 hour multitasking day.