Explanation: NAVTEX broadcasts operate on Medium Frequencies (MF), primarily 518 kHz and 490 kHz. During the day, the sun's radiation creates the D-layer in the ionosphere, which strongly absorbs MF signals. This limits daytime propagation primarily to ground waves, resulting in shorter ranges. At night, particularly in the middle of the night, the D-layer dissipates due to the absence of solar radiation. Without the D-layer to absorb them, MF signals can travel higher into the ionosphere (to the E and F layers), reflect, and return to Earth. This phenomenon, known as skywave propagation, allows signals to travel much greater distances, significantly increasing the transmitting and receiving range of NAVTEX broadcasts. Therefore, the middle of the night provides the optimal conditions for maximum range.