I only know realized this, but them having numbers on their eyes is a pun: 1st, 2nd, 3rd, etc. are written as ichiban me (一番目), niban me (二番目), sanban me (三番目), etc.
In this case, "me" expresses numerical order, but when alone, it means "eyes".
@TrueGoddessReincarnation Most Japanese homes have room heat/ac, not central, and electricity prices in Japan are pretty high compared to North America or Europe. So they heat/cool rooms as needed, and using a kotatsu means you don't have to pay to heat the entire room. Also Japanese homes tend to have really poor insulation, so they're drafty in winter. Check out Life Where I'm From or Rachel and Jun, both on Youtube, for some up-close looks at Japanese houses.