For me, the biggest factor seems to be whether or not I've had enough sleep. If I haven't, my eyes get sore, my brow feels tense and text has a "shimmer" to it.

For me, the biggest factor seems to be whether or not I've had enough sleep. If I haven't, my eyes get sore, my brow feels tense and text has a "shimmer" to it. A nap is often enough to fix it.