I know what you mean because it happens to me. When I am doing a big meander sometimes the thread disappears. I have to stop and check, but then of course I can't maintain a curve so I get a little directional shift.
I have an Ott light and I turn it off because it is so shadow-free and I use a big halogen desk lamp on a long arm, like an architect's desk lamp. I pull it down close to the work and it casts a bit of shadow. I do that whenever I need to see something 'really good'.