Why is long-distance communication on the 40-, 60-, 80-, and 160-meter bands more difficult during the day?

The D region is the lowest layer of the ionosphere and is heavily ionized by solar radiation during daylight hours. Unlike the higher F layers which refract (bend) radio waves back to Earth, the dense D region tends to absorb lower frequency signals, particularly those on the 40-, 60-, 80-, and 160-meter bands. Because these longer-wavelength signals must pass through the D region to reach the refracting F layers, they lose most of their energy to absorption during the day. After sunset, the D region quickly dissipates, allowing these lower frequency signals to pass cleanly to the F layers and propagate over long distances.