How long can you focus mind on your jobs everyday and When?

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Although we have to work for 8 hours every day, we may only spend less time focuing mind on jobs. For me, I start work at 9:00 am and just feel so sleepy for the next 1 hours and then, I feel concentrated at 10:00 when I have a great efficiency on jobs. But such an efficiency can only last 30 minutes...Then I feel tired from 10:40 until 12:00 because I will have lunch and feel relaxed. In the afternoon, again, I feel sleepy between 1:00 and 3:00, then I get awakend and have another concentration on 3:30. It also only lasts 30 minutes. So the the duration that I involve in the jobs totals only 1 hour. How about your time spectrum? Can you focus for 8hours?