Explanation: NAVTEX broadcasts operate on medium frequencies (MF), primarily 518 kHz and 490 kHz. The range of MF signals is significantly affected by the ionosphere. During daylight hours, the D-layer of the ionosphere, which is created by solar radiation, absorbs MF signals, limiting their propagation primarily to ground waves. At night, particularly in the middle of the night, the D-layer dissipates due to the absence of solar radiation. This allows MF signals to penetrate to the higher F-layer of the ionosphere, which then reflects them back to Earth. This phenomenon, known as skywave propagation, dramatically extends the range of MF transmissions, often allowing them to be received hundreds or even thousands of miles away. Therefore, the "middle of the night" offers the optimal conditions for maximum transmitting range.