This phenomenon, called 'skip' occurs as the atmosphere immediately above the earth cools down. High temperatures act as an impedance to radio waves, and lower temperatures just the opposite. If the air is cool enough, radio waves will literally 'bounce' off the ionosphere to points out of sight, making a bend around the earth. This is accentuated in summer months when there is warmer air in the troposphere, about 7-10 miles up, that keeps a colder-air 'duct' below. So under these conditions radio waves will travel remarkably far. This is why more powerful AM radio stations are licensed a 'cleaar-channel' at night, for they will simply cause interference to local stations on similar frequencies.
They don't travel faster, but they do travel farther.
Ditto Bickle. I have noticed that phenomenon, when traveling through the desert States. I thought perhaps the fact that there are fewer communications being sent through the air, [during the day] by radio stations, T.V., and cell phones, Also less microwave oven usage and such during the day, might factor in.
I think it is because most people listen at night rather than day.