Goose down can be found on the chest and stomach of the goose. The down is hidden underneath a layer of feathers that acts as a waterproof barrier. This layer protects the down from water, which allows air to become trapped between the down particles. The down particles, made out of little bifurcations, hold on to that air.
The down particles that hold onto air create a layer of insulation, that holds on to the body temperature of the goose when she’s in the water, in snow and everywhere in between. It is therefore not the down particles that are warm, but rather a network of air and down particles that hold onto body temperature. And it works the same way with humans, so you’re really sleeping in a cocoon of your own warmth.
This means that a heavier duvet doesn’t necessarily mean it’s a warmer duvet. The bigger the down particles, the less of them you need to fill a duvet. Bigger down particles can also hold on to more air, as they have more bifurcations which creates a better layer of insulation. A lighter weight duvet can thus be warmer than a heavy one.
It is therefore important to know whether you’re a hot or a cold sleeper, because no one enjoys waking up in a sweat! If you get cold easily, you’ll need a better layer of isolation that can hold on to your body heat well. If you tend to run hot you’ll need a duvet that allows some body heat to escape.