Our working day, while we were in Brazil, was dawn to dusk -- about 5.30 am to 6 pm. We were up and out by 5.15 am, watching the light rush over the landscape, as it tends to do in the tropics (no languid, leisurely dawns there), then we headed to breakfast at about 6.30 am, feeling like we'd already accomplished something. Then there was the relatively cool, productive period until about 11 am, an agonising hour when our stomachs rumbled incessantly for lunch, followed by the flattening, oppressive heat of the early afternoon until about 3 pm. The final stretch from 3-6 pm was pleasant, gradually cooling, and with a lovely golden light cast on everything.
I mention this because I found it quite a pleasant way to work. There's something just right about timing your working day to match the available light. You get a feeling of continuity as you watch the bats (which you saw leaving their roosts at the end of the previous day) returning from their night time foraging, and the cormorants and herons (which you saw coming back to their roosting trees the previous evening) leaving to start their day. Their activity synchronises with your activity, and after a bit of adjustment to the early start, you find that it sits very comfortably with the natural changes in your energy levels.
Of course, if you get up at 5 am, you have to go to bed at about 9 pm at the latest (though we rarely lasted past 8.30 pm), so it somewhat curtails your social life. And in temperate latitudes, your working day would oscillate wildly between unworkably short in winter, to exhaustingly long in mid-summer. Of course, this is how people used to work when their calendar was driven by the agricultural year, and artificial light was expensive and hard to come by. Explaining to your boss that you are late to work because it isn't yet light is going to sound like the lamest of excuses now. But if I had more freedom to schedule my own day, I'd like to follow the periodicity of the natural day length more closely -- I think that I'd have a lot more energy to spare.