There is a lot of good advice here, so I will try to add a few useful thoughts and experiences.

The amount of useful work you can do in a day is unique to you as an individual. Some people have more mental "stamina" than others, just like some people can run marathons and others can barely get to the end of the street. I think therefore, the first thing you need to work out is how much useful work can you do in a day and don't go beyond this for sustained periods. Trying to push yourself well beyond your current limit can be a vicious cycle as over working will make you tired which will lead to errors, which will lead to rework. This can lead to the odd situation of needing to work 12 hours a day just to do what you did in the past in 4 or 6 hours.

If you want to extend your capacity for productive work, I would consider the following:

• Never work for more than 50 minutes in each hour, take a break, walk around, do some light stretching or exercise
• Be aware that your alertness and ability to concentrate varies throughout the day. Some people are at their best early in the morning, others late in the evening. Everyone feels drowsy in the early afternoon after a heavy lunch. Aim to take on your toughest tasks at the time you are most alert.
• Drink plenty of water, being dehydrated leads to fatigue
• Do some exercise during your working day it will break up your time at your desk and release endorphins
• Enjoy some fun time with your friend and/or colleagues every day playing computer games, table football, table tennis, etc. This relaxation time will help recharge your energy.