It is normal that your productivity is not the same during the day. Your strategic task is to achieve a long-run ability to work.

It is wrong to try to work until you get a headache. If you save health, you also save your productivity. You have to set and follow working hours and resting hours as well.

The following rule should be established: if you are tired, you have to get some rest.

Rest can be active or passive. After some active rest (playing football or a computer game), you can get a better mood and a motivation to work. But some passive rest (sleep) is needed to restore the vital forces of your organism. That's why you should combine active and passive rest.

Planning ahead resting and working hours can be recommended as very good tactics.