The release patch added a work speed buff for working in an area with light. For example, putting the research station next to the printing pod will make your research faster because of the light coming from the pod. As an experiment, I put a lamp next to an outhouse to see if it made a difference and it didn't seem to.
EDIT: it does affect outhouses! see posts below.
Is there any way to find out what activities are affected and which ones aren't? The buff seems to show on the duplicant in a lit area regardless of whether they are doing anything or it is having any effect. I'm pretty sure things like research or smelting are affected, but what about "non-machine" activities? Eating, showering, bathroom, harvesting plants, grooming critters, putting on an exosuit, using the jukebot/video games - it would be good to know which ones benefit from lighting.