... let circulate the air underneath  you.
The horsehair is an aired and hollow body. Giving a perfect structural strength, it has also the perfect capacity to absorb and to evaporate the humidity. It is shaped in strong spirals to optimize its elastic properties. A slow circulation of air constantly settles in the stuffing and constantly  new air is brought to the body. The mattress in horsehair is strongly recommended against all orthopedic and rheumatic pains or sufferings ...

The sleeper's perspiration must be absorbed by the stuffing. The horsehair can absorb 20% of its dry weight in humidity without becoming humid on the outside, besides it is capable to clear the humidity toward  the ambient air in case of saturation. No other stuffing material possesses such a capacity. The horsehair in the mattress encourages the coetaneous breathing and therefore the blood circulation.