When we were hunter/gatherers we were constantly active, so it wasn't exactly an issue. However, with the sedentary lifestyle most people tend to have these days, we need to regulate our bodies. Eating and sleeping on a regular schedule is a big part of that. We aren't hunter/gatherers now. Nor does the fact that, during part of human history, we were hunter/gatherers mean that our entire system is tuned to that evolutionarily. Remember, not all evolutionary traits are beneficial, and if one trait is present but, as in the case where during that time we wouldn't need to worry about how our body responds to a sedentary lifestyle, the trait isn't overtly harmful, then it won't necessarily be selected against either.