Light in a room is not a single layer.
It is not. The light you need to walk across a room and the light you want when you sit are not the same job. Asking one ceiling for both does not make the room better.
What is a layer?
A layer is a separate job light does in the room. GE Lighting names three: ambient, task, accent. Interior practice adds a fourth: decorative.
Ambient light lets you walk the room. The Illuminating Engineering Society calls it a spread environmental light. Task light sees a desk or a book. Accent pulls a painting or a wall texture forward. Decorative light, like a chandelier, shows itself as well as lighting the room.
The ceiling is for walking. Light spread on a wall is for sitting.
Why does it not all come from the ceiling?
The ceiling is the most common way to build ambient light. It is not the only way. The same job can be done with light spread on a wall. When the ceiling is fully on, every surface looks the same. You can walk. Where you sit does not sit apart.
Changing the colour of the ceiling bulb does not change that job. The light still arrives from above. A strip stuck to a wall paints one surface. It does not hold the room.
Where else can ambient light come from?
From a corner, toward a wall. You see the light on the wall, not the source. The room may be dimmer than it is under the ceiling. Sitting in one spot is still easy.
That ambient light is our job. It does not replace a desk lamp or a picture light. It does not replace the ambient light in the ceiling either. Ambiance is how this layer sits in that moment. The definition is in what is ambiance?.
What happens to the other lights?
A lamp for a book and a source for the table stay in place. Opening the ambient light does not switch them off. If you leave every light fully on at once, the layering is gone. The room looks like one light again.
What light does to alertness is a separate subject. It is in does light actually affect our mood?.