Most lights try to avoid flicker in the design, but bad/failing ones can drive one nuts. E.g. fluorescents flicker at 120 Hz (2 crossings per cycle) but the phosphor coating carries some glow and that they stay on for ~80% of the cycle time anyways makes it a lot more bearable that if it were an idealized instantaneous 50% duty at a 60 Hz rate.

God bless LEDs with some capacitance though...