Back in the States. I turned the small hand of my watch back five hours. Reset the clock in my laptop. But I still woke up at 1 AM. I know from past experience that it will take weeks to reset my body clocks.
"We live in an old chaos of the sun, or old dependency of day and night," wrote the poet Wallace Stevens For billions of years, a sensitivity to light and darkness was key to survival. The whole point of plants is to soak up sunlight. Animals too evolved patterns of hunting and feeding in light or darkness that increased their ability to find food or lessened their vulnerability to predators.
The earliest mammals may have been burrowers in dark tunnels, with a world of hungry dinosaurs overhead. Those troglodyte ancestors needed to know -- needed to feel in their bones -- when it was safe to go outside. Any genetic mutation that finessed their circadian clocks was presumably favored by natural selection.