system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is it that when it's dark you see with less detail on your fixation point and have to use the area around it?
gpt
There are two kinds of light receptors in the eye, rods and cones.  Cones give you color perception and work better with higher light levels.  Rods work better in lower light levels and for detecting motion (because they respond to light changes faster) but aren't sensitive to specific colors.

The center of your field of vision is mostly done with cones and the periphery is mostly rods.