Human beings are built to respond to the natural light and dark cycle of the day. It makes sense that we would have evolved this way. After all, unlike cats, our eyes aren’t equipped to see very well in the dark. So, we had to do our hunting in the daytime and, the earlier we started looking for food, the more food we’d find before the day was over. As a consequence, our brains are more alert during daytime hours. As the sun sets, we start to slow down. Even if you consider yourself a night person, your productivity still wanes with the loss of daylight. Of course, nowadays we are surrounded by artificial light. It makes sense that this light might wreak some havoc on our sleep/wake patterns.