Being in a situation where I make my own hours, I started wondering if 8 hours a day is the most productive way. Enough to do a lot of work but not too much so that your brain goes numb or your body fatigues. I decided to do some research on the subject and this is what I found.
Around the industrial revolution in Britain, the workday was anywhere between 10-16 hours a day. Robert Owen (http://en.wikipedia.org/wiki/Robert_Owen) coined the term coined the slogan "Eight hours labour, Eight hours recreation, Eight hours rest." and pushed for lowering the hours a day people worked.
Here (href="http://www.lifehack.org/articles/lifestyle/are-you-working-harder-or-working-smarter-looking-at-the-40-hour-work-week.html) is a good article on many people's philosophies of the best number of hours to work a day/week. From what I can tell the 40 hour work week is just an arbitrary number of hours chosen because it was less than 80.
Is working 8 hours in a row best? I know that for me personally when I start working in the morning through lunch, I am very productive but there is a noticeably less intense work ethic right after lunch. Perhaps lunch would be better extended to 2 or 3 hours? There are some physical reason why we would slow down after lunch as the digestive system takes a lot of energy, but I also think there are very psychological reasons something different than an 8 hour work day might be more productive. If you have 1 hour to get your house clean before guests arrive you'll be very productive, but if you got to work with 8 hours to kill, you'll tend to drag things out as long as you can to fill the day. If we were in a job where we could get all hour work done in a couple hours and go home I think most of us would prefer this, assuming the pay is the same of course.
I think most of us are in a position to try to drag out our day instead of making it as productive as possible.
There is a psychological aspect that comes into play as well. If you spend too little time working you wont have any sense of accomplishment. I know if I don't feel like I've accomplished enough I get depressed. On the other end of the spectrum if you work too much you'll become stressed which is also not healthy.
I've tried different methods of timing myself, for instance starting a stop watch and making sure I am productive for 8 hours in a row. Other times I've tried using the stop watch just for an hour at a time but trying to get at least 4 hours in throughout the day. With the 8 hour system, as I said before, one tends to slack off more, and drag out tasks to fill up the time. The 1 hour system works well because it's not hard to make 1 hour bursts of productivity 4 times a day actually get more done than trying to do 8 straight. The problem with that method however is that often after the first or second hour, the third and fourth never come to fruition as you may get distracted by something more recreational.
After trying these two methods, and several variations on each, I think the most effective way to get the most done in your day is not to pay attention to hours at all. I've found a task based method to be more effective, every morning writing out a to do list.