Having looked at a computer screen, it takes a while for the eyes to refocus when you look away toward a distant scene outside your window. (Hopefully you have that window looking over some distant scene, if not, try to have something further away, that you can turn around to look at. A picture on the wall, behind you, a flower vase across the room, for instance).
The reverse also applies. If you have looked at a bright, distant object and then shift focus to a book, your eyes will take time to adapt.
The brighter the first object viewed, the longer it takes to acclimatize to the new object.
This applies to tasks we carry out when we are multitasking. Hence the ability to multitask with minimal inefficiency is due in part to the “brightness” or mental intensity of the different tasks we are juggling. Generally, I try to avoid multitasking, but that is not always possible to avoid in today’s complex life.