Gemara: What is bein hashemashot (halachic twilight)? From the time the sun sets, as long as the east side of the sky has redness. When the bottom side has darkened and the top has not, this is still bein hashemashot. When the top part darkens and is the same color as the bottom, this is night. The preceding is the opinion of Rabbi Yehuda.

Ein Ayah: The phenomenon of day and night and the transition and relationship between them reminds us of the approach to historical periods in the annals of the Jewish people that can be compared to day and night. Day represents times of liberation and glory, and night represents times of deterioration.
The fundamental difference between these periods is that in the time of greatness, matters relating to the collective have the greatest prominence, and all individuals find happiness and pride in the success of the collective. A situation of "daylight" means that the actions of society are so great that they swallow up the tendency toward individualism.
When the nation declines, destruction sets in. The people are scattered, their centers are destroyed, and the spirit of the collective is lost. "The sun sets, and night falls." Elements of national life disappeared when our Glorious House was destroyed and our unifying capital city lost its centrality. One needs to define the extent to which the national spirit declined in order to set new boundaries between public and private life and calibrate their interaction.
This is important because even after the kingdom ceased, the Temple was destroyed, and the nation was scattered, Hashem still preserved for us a situation in which there were rays of light and the "eastern sky had a rosy glow." The nation still remembered well its glory, and its connection to their collective successes lived powerfully in its heart.
In the first stage after destruction, the individual felt the loss, even in his personal life in regard to his material state, so that this part of his life could not be complete and full of light. However, in the element of his spirit, there was still a connection to the days of glory, and there was still a connection to the era of liberation.
As time moved on, any connection to the time of independence and national success disappeared, and the last vestige of connection to the national era was lost in the lower realm. However, as long as the individual still felt the spirit of strength and felt a connection to the spiritual grandeur that existed in the upper realm, it was still bein hashemashot. There was a mixture of exile and liberation interplaying within the person.
Once even the upper, spiritual realm only had a connection to the individual, not the collective, this already signaled the era of night, of exile. All of these matters are a function of time. As more time goes by from the previous period to the one in which people are now living, the connection lessens. Over time, the dominion of the collective is darkened, as this element of life is lost, and the life of the individual, with its focus on matters of little significance, takes its place. Then darkness of full night becomes fully palpable.