Howto have dark entity when no sun illuminated


I’m new to cesium and I’m impressed about how powerful it is.

I’m doing some tests with sun illumination and I have a silly question.
I Would like an entity (sphere in my case) to get dark/black when the sun disappears, but I’ve not found a way to do it. When the sun sets the sphere gets brighter.

Wonder if it is a material setting, but was not able to find it. Please can anyone help?

Here is my simple sandcastle demo code

var viewer = new Cesium.Viewer("cesiumContainer", {
  terrainProvider: Cesium.createWorldTerrain({
    requestWaterMask: true,
    requestVertexNormals: true,
  shadows: true,
  terrainShadows: Cesium.ShadowMode.ENABLED,
viewer.scene.globe.enableLighting = true;

var sphere = viewer.entities.add({
  position: Cesium.Cartesian3.fromDegrees(
  ellipsoid: {
    radii: new Cesium.Cartesian3(100, 100, 100), 
    material: Cesium.Color.WHITE,
    shadows: Cesium.ShadowMode.ENABLED,

viewer.zoomTo(sphere, new Cesium.HeadingPitchRange(0.5,-0.3,1100));

Thanks in advance for your help