A PNAS article about the use of e-Readers is often cited [0]. However, this compares a light emitting device to a book, and it's not clear how applicable it is to Night Shift vs. not Night Shift.
There is some research about the affects of light at different wavelengths on the human sleep cycle but I'm wary about using that as evidence for the efficacy of something like f.lux or Night Shift without specific studies done on their use in practice.