A very generic question, as there are many factors that have to be taken into account. There are personal and work related factors. Examples of personal factors are: "How much are you enjoying your work?", "How long can you stay focussed while working on a task"?, "What's your sleep schedule like?", "What did you do on the weekend?" or "What's your eating pattern?". Work factors are: "What type of work are you doing?", "What's the company's culture like?", "Are employees keeping each other motivated?" or "Are you working on a task alone or as a group?". All these factors have an impact on the outcome of the question, whether working 8 hours works well for you or not.
Some of these factors are variable and therefor I can say from personal experience, that each day is different. When talking about "productive hours", is it possible to work 8 hours a day on regular working day? Yes and no.
No, because there will always be random events, that interrupt your work and lead to a decrease of productivity, such as: a colleague that needs your help or unplanned meetings. Given the examples, does that mean your inproductive? You're not finishing your tasks maybe, but you are helping the company, so you are in fact being productive.
And yes, because you can improve your life in a lot of ways, for example by patternizing your daily activities, so that every regular working day is exactly the same e.g.: strict sleep schedule, healthy diet, daily walks outside when you have an office job, limit caffeine intake to the mornings and so on.