The antonyms for the phrase "letting sunlight in" would be blocking, obstructing, or shading the light. When we talk about the opposite of letting sunlight in, we mean keeping out the light altogether or limiting it in some way. This can be achieved through blinds, curtains, shutters, or any other materials used to create shade. While these measures may be necessary to protect against harsh UV rays or maintain privacy, they can also create a dim and gloomy atmosphere. Therefore, it's essential to strike a balance between letting enough sunlight in for optimal natural lighting while also maintaining comfort and safety indoors.