I'd suspect neck tension first, a high pitched sound from the monitor second (check with a high quality mic recording and spect analysis, since you may well not actually be able to hear it), and as a distant fucking third idea I would think about "how do EMFs interact with the human nervous system", but I think that last idea is one hell of a stretch based on the available science.
I sit in front of a combination of two 24" 2k monitors @ 120hz, and one 27" at 144hz, for ~4-12 hours a day. The times I notice spikes tend to relate to posture and jaw/neck tension and lactic acid buildup.