It's a natural part of autumn -- twilight comes earlier as the amount of daylight decreases.  That trend is hard to ignore at this time of year, in particular, because we're losing more light each day than at any other time of the year.

The graph above shows the amount of daylight each day in Minneapolis over the course of a year.  At the summer solstice (in the middle of the graph) daytime is long, nighttime is short, and the rate of change (represented by the red line) is minimal. 

The rate of change is also minimal at both ends of the graph (which represent the winter solstice); however, at this time of year, daytime is short and nighttime is long.

The green lines on the graph show the rate of change in daylight during the spring and fall; specifically, near the vernal and autumnal equinoxes.  Those lines slope more steeply, indicating a change in daylight of about 3 minutes each day!

That brings us to this week (and next).  As you can see from the chart, from Monday Oct. 9 to Sunday Oct. 15, we'll lose a total of 15 minutes of daylight!  That's a lot, and definitely noticeable.

So, why does the amount of daylight change at different rates during the year? The tilt of the earth's axis is the culprit; if our axis didn't tilt there would be only one season, and there would be 12 hours of daylight all year round. How boring. For more, watch Heather Brown's excellent "Good Question" segment on this topic!