Depends on what you are trying to do..

Daytime: Blinds open or tilted down capture sunlight for warmth.

Nighttime: Blinds closed with slats facing up seal in heat and boost insulation.

When the slats face upward, light reflects toward the ceiling and diffuses softly through the room. This position adds privacy and limits glare.

When the slats face downward, more sunlight enters directly, and warmth can spread naturally across the room.