How do you do it fellow developers? How can you stay productive for the whole day? I've tried and best I can do is 6 hours without feeling my brain mushing. Why do management insist I need to sit in front of the computer for the whole 8 hours...

You don't. Humans cannot physically stay focused and active for several hours in a row.

Take breaks.

Your management probably consists of idiots 🤷‍‍♂️

Its just a contract thing. 6 hours is good work.

I put too much weight on hours and all it got me was back pain, some burnout and a habit of procrastination. The problem is that theoretically your employer has leverage over you when you don't work the hours stated in the contract, especially if your performance is on the low.

In practice your manager should (and often will) be much more happy about 3 to 4 hours of concentrated work leading to results than 8 hours of twiddling thumbs. Unless they're idiots.

@theuser not until the micromanagement kicks in... So far my relaxation consists of hot stack exchange questions and reading newsgroups

Iirc 3 hours of total focused work was average.

I was on the job that consisted of errors that were hard to troubleshoot: at least 1-72 hours spent trying to dissolve each.
That must be pretty idiotic, but imaginary pressure was holding back my hours from the real task, because of the shit of the system we integrate.
JFC, I'm glad I left it all after 1 year there. No regrets.

i tried working for more than 12 hours, i could feel the back of my eyes pulsate