Check flicker-free on Wikipedia:
Contrary to popular belief, modern LCD monitors are not flicker free, since most of them use PWM for brightness control. As the brightness setting is lowered, the flicker becomes more noticeable, since the period when the backlight is active in each PWM duty cycle shortens. The problem is much more pronounced on modern LED backlit monitors, because LED backlight reacts faster to changes in current. Many users have reported headaches, eye strain, nausea, and other problems.