Lumen cannot function without an active light source, since it relies on active light sources within your world to calculate Global Illumination and Reflection. Light sources cast “rays” into the world, and when a ray of light is cast from a light source, such as the Directional Light, it hits the surface and then "bounces " off it, with each bounce transferring some lighting information to and from various surfaces.
“Lumen Global Illumination solves diffuse indirect lighting. For example, light bouncing diffusely off a surface picks up the color of that surface and reflects the colored light onto other nearby surfaces — creating an effect called color bleed. Meshes in the scene also block indirect lighting, producing indirect shadowing.”
Your scene becomes “completely dark” likely because there is no light source (such as the moon or lights) when the sun goes down for Lumen to use when calculating GI or reflections within the scene. The distant light is below the horizon, so its rays won’t hit the surface. If you add a secondary source of light (maybe a moon or some lights) to illuminate the area once the directional light goes below the horizon, it should potentially provide Lumen with the ability to light your scene.
The Dev page has some good documentation on Lumen to help you understand it, and Lyra Sample Project also showcases Lumen quite well!