Dehydration is linked to the deterioration of the skin's barrier function. The causes: hormonal changes, cold, wind, smoking, etc. To avoid damaging the skin's protective film, gentle hygiene is important: use soap-free cleansing products, avoid hot water and overly vigorous rubbing. Apply a moisturiser for your skin type every morning, a moisturising sleeping mask to boost hydration at night, and a moisturising serum at each change of season.