The first thing you notice on Sonabia 2 is its color: Hull and deck are a dull dark grey. Sonabia 2 looks like a tank.
Explanation: heat with solar radiation during the day, release the heat slowly during the night. See below.
The external skin of the hull is a matte dark gray, it has a low albedo. It absorbs about 75% of the solar radiation and reflects only 25% (these figures are only illustrative). Hence about 75% of the solar radiation is absorbed by the hull which in turn radiates heat into the cabin.
The internal skin, light grey and polished has a high albedo. About 75% of the infrared radiation (heat) coming from the cabin is reflected. Only 25% of the energy escapes through the hull. The insulating hull material (thick PVC foam) reduces further the heat loss. In practical terms, by day the cabin warms up and it cools down slowly after nightfall.