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Sleep mask affects gameplay, if put on decreases volume (slightly but noticeable)

light doesn't affect gameplay except it makes your screen brighter
X-ray is just for those who want to see the process

The brightness of the light doesn't affect anything, it's purely a visual effect for the atmosphere. The same goes for x-ray, a feature that makes the process itself sweeter. 

As for the sleep mask, it affects the sleep sensitivity indicator (the strip at the top). With the mask on, the girl reacts less to your actions.