Of course one could use a vent but this is not feasible for me. Is there a low-tech solution, like some kind of a specific curtain? I could also cover my window with an opaque board, letting some space but this lowers dramatically the quality of the air (mainly CO2 wise).
3 layers of pegboard (pierced hardboard), internal surfaces painted black, with the middle layer offset, and spacers between the layers. This should provide little or no straight path for light, but allow ventilation.
Consider a combination of options:
Awning over the window. This keeps sunlight from striking the window at least at mid day. The wider the awning is, the more time it shades the window.
Exterior bamboo rollup blind. This will block about 90% of the light. If you mount it so that it stands a few inches out from the wall, it won't impede the air much at all.
Miniblinds. The cellular ones block too much air flow. Get the blinds is as dark a colour as you can. (Dark blue?) In use angle the blinds so that light must reflect upward to get in. This still allows good airflow, but no direct light at all.
Double drapes. You can get nested curtain rods, with the inner set being about 2" from the wall, and the outer set about 4". Traditionally they are used with sheers on the inner layer to let in light and air, but still give privacy, and a heavier drape used at night in winter and made for reducing drafts. (Good windows are a recent invention) But there is no reason you couldn't use some breathable fabric that was dark.
Screen. A folding screen that stands a foot from the window with the back side painted flat black, or if you can afford it, vanta black.
Light reduction is multiplicative. But the human eye is sensitive to a huge variation in light. Normal office lighting is about 1/10,000 that of sunlight.
Have you considered a sleeping mask?
A black painted louvre, if you still get too much light, add another behind it with the slats slanted the other way.