8 hours of sleep can undo less than 4 hours of problem-solving activity. Our large brains are already consuming too much body resources.

I have CO2 monitor, which I use to estimate is my brain working or not: when I'm working, CO2 level raises quickly above 1000ppm, so I need to ventilate my room often; when I'm idling, e.g. by playing a simple game, CO2 level is at about 740-790ppm without ventilation for whole day.

When I'm returning from work, I see at CO2 monitor that my head is still working. Yes, I'm able to «work» sustainable 4h per day only, but «work related activity» consumes my brain for whole day.