This is about LED backlight flickering, that is on every Galaxy Note 2 and many other PC monitors, TV screens, mobile phones that have LED backlight.
Google "led flicker display" and you can see many forums that talks about eye strain and headaches caused by LED flickering.
You could have headache, but not realising that is from using your phone every day or monitor.
For better understanding:
http://www.infobyte.hr/blog/134/ccfl-...
Don't get me wrong, this is the best phone that I've had.