Twice a day, at sunrise and sunset, the ionosphere offers enhanced propagation.
The D and E layers absorb radio waves that pass through them, but the absorption is frequency dependent. The D layer can completely absorb signals on 160, 80 and 40 metres during the day and can attenuate signals on 20m. That's why you don't hear much DX on the low bands during the day as sky-wave signals are absorbed before they can reach the reflective E and F layers.
The ionosphere changes at the transition from day to night. At sunset, the D layer disappears rapidly. Around the other side of the world other regions that are entering into daylight have yet to form any significant D layer and the E layer has not built up from its night-time low.
For a short period propagation between two regions simultaneously experiencing sunrise and sunset can be highly efficient. Signals on the lower bands can travel over great distances with little attenuation.
We know that D layer absorption is inversely proportional to the square of the frequency. This means that in practice grey-line effects should be more pronounced at 160m than say 80m and even less evident at 40m. By the time we get to 20m and above D layer absorption is not the major factor it is at LF. That's why you can't generally work DX on 160m during the day and yet 20m can be wide open.
The problem is it's hard to tell if your perceived better reception is down to the grey-line or just better conditions when you chose to listen. Even if you can guarantee your signal is coming in via the grey-line, it will only be an S-Point better, some 6dB up.
At the end of the day and the grey-line does happen at the end of the day, you have to ask yourself if it's worth all the effort. Enjoy it when it happens.
The region of the ionosphere known as the D Layer is at an average height of about 60-90km above the Earth's surface. It is mainly responsible for absorbing or just attenuating signals on 160-20m. This is why we don't hear much skip on the LF bands during daylight, especially in summer.
The actual level of absorption is dependent upon the frequency, time of day, time of year and solar/geomagnetic levels.
According to the RSGB Amateur Radio Operating Manual, for a daytime path:
6dB attenuation per hop on 21MHz - about 1 S-point
40dB attenuation per hop on 7 MHz - about 7 S-points
100dB attenuation per hop on 3.5MHz - about 17 S-points
This projects to around -250dB per hop on Top Band which nicely explains the daytime silence. Who would put up with a 40 S-point loss?