Shabbat occupies the daytime rather than evening and night. In the Creation, God worked during the day, and then “there was evening, and there was morning,” and the next day of Creation. The seventh-day rest, which is opposed to divine work on the previous six days, applies to daytime: just as work was performed during the daytime, so the symbolic no-work is also observed in the daytime.

The sixth day of divine work is concluded with, “And there was evening and there was morning” (Genesis 1:31). Then the work was finished (2:1). Then (consecutive “and”): “And He rested on the seventh day from all His work.” So the rest took place after the morning. The Torah doesn’t end the description of the seventh day with the customary, “And there was evening.” The rest started after dawn and was concluded no later than the evening. Shabbat, the likeness of the divine seventh-day rest, by analogy applies to daytime only.

Gen1:31: “And there was evening, and there was morning—the sixth day.” The previous day ended at dawn, and on the seventh day, Shabbat ensued. The biblical day lasts from dawn to dawn.
“It was evening” always comes after the list of created things. So the creation took place only in the daytime, before evening. On the seventh day, God rested. Abstinence from work was only observed in the daytime; in the evening work was never done, and so was not abstained from on Sabbath. Therefore, Sabbath starts at dawn and continues until evening.

Any other reading creates a lot of problems. The prohibition of having fires on Sabbath would make the cold, lightless nights in the desert anything but joyful if the fire started earlier went out for some reason. The Karaites, indeed, sat in darkness on Shabbat, while the Rabbanites creatively reinterpreted the commandment, understanding it to apply only to kindling a new fire. It is easy to remain without fire and refrain from work on Saturday after waking up, but burdensome to make sure to end all work in the nick of time before dusk on Friday.

In primitive economies, people were not expected to work at night because lighting was scarce. Friday evening’s prohibition of work would be a tautology.