Atmospheric refraction bends the light rays from the Sun by slightly more than half a degree. This bending causes the Sun to be in an apparent position higher than it is in reality. The effect is a somewhat sooner sunrise, with the upper rim of the solar disk appearing (on average) five minutes before the expected time.
The bending of light rays is accentuated at sunrise and sunset since the distance traveled in the atmosphere is longer, and the angle of incidence is higher.