Always start with Dial a day.

Gen1v3 And God said, "Let there be light," and there was light and God saw that the light was good, and he separated the light from the darkness. God called the light "day", and the darkness he called "night". And there was evening, and there was morning - the first day.

The earth cooled and so did the worldwide ocean. This reduced the rate of evaporation. The cloud layer thinned. There was still much carbon dioxide in the atmosphere that absorbed a lot of solar radiation. However there was less water vapour and fewer cloud droplets so some light was able to penetrate to ocean.

It would be interesting to know which wavelengths of solar radiation were most likely to penetrate to the oceans under these conditions. Short wave radiation is most penetrative but also the most easily scattered. The existence of short wave radiation was important to produce the small quantities of oxygen in the atmosphere necessary for the origins of life. It is also thought that the chemicals in the oceans upon which life is based were produced under the influence of short wave radiation.