Average American lives at 21°C year-round. Winter — heating. Summer — AC. Body doesn't know the season.
Amish
Winter — cold (wood stove heats kitchen and living room, bedrooms cool, sleep under quilts). Summer — hot (open windows, tree shade, early rise while cool). Spring and fall — perfect.
Health
Research: people exposed to temperature variation have better metabolism, sleep deeper, get sick less. Amish confirm: obesity 4%, diabetes below average, depression rare.