Squalane

Squalane is the stable form of squalene, a lipid the skin already makes and makes less of with age. The -ane is the whole difference: squalene oxidises, squalane has been saturated so that it does not — which is what makes it usable in something that sits in a sachet before it is worn.

Where the humectants hold water, this is the part of the formula that slows the rate at which it leaves. The two do different jobs, and a mask worn overnight needs both.