As a remote employee I never worked more than 8 hours a day (including lunch break), knowing my friends in the office sit there for 8 hours INCLUDING lunch breaks, table tennis sessions, kitchen chats, multiple smoke breaks, meetings, scrum sessions and so on.
Also I know very great programmers and literally NONE of them can stay really focused for more than 5-6 hours a day, regular devs will struggle to be productive for 4-5 hours, I'm happy if someone actually works for 3-4 hours a day. Sitting or standing in front on computer for 10 hours a day is a highway to carpal tunnel syndrome, haemorrhoids etc.
If they say 8 hours (full work day), they're either lying or using some kind of performance enhancing drug.
I've been paid for software development for close to 20 years and I've never done anything productive before lunch. Mornings are for documentation, code reviews, emails, stuff that doesn't really need constant focus. Afternoon and early evening are when I get shit done, that's when I can get into the Zone.
The point was that being "productive" - that is to say fully focused - for a complete workday just doesn't happen all the time without medication of some sort.