The side of the mattress is mostly just flat, so unless the light source is nearby and weak enough to have quick fall-off (e.g. a candle, a table lamp), the side would actually be largely flat-coloured, aside from shadows cast on it by the sheets and ambient occlusion shadows (shadows created by crevasses which light has a harder time getting into) from the details/texture on it.
Shadows are just areas where there is less light. So, to draw shadows, think of the object as a 3D thing, don't reduce it to a cardboard cutout. Imagine your light source and your 3D object. Where does the light reach, where is its path blocked?
Some very rough edits to show you how the bed might look lit from various directions:
The top row has the sun, a very powerful light source with no noticeable fall-off.
The other rows have a candle or nightlight with very noticeable fall-off.
The 3D position of the light matters. In all of these cases, the light is just slightly in front of the bed. If it were behind, you might get something more like this:
I got rid of most of your texture when making these edits because I wanted you to focus on light in general. You can always add the texture later. At this size though, you should keep it to a minimum, focusing on just the largest folds. Small texture in pixel art tends to just look like noise.
If you struggle with imagining things in 3D, I recommend drawing the bed at an angle where you can see more than just one side of it. That might make it easier to understand what's going on with the light. You may also want to do some exercises in shading simple objects,
like this.
