The idea that your skin works while you sleep isn't marketing - it's biology. Korean overnight masks are built precisely on that: instead of fifteen minutes, they stay on the face all night and work with the body's natural rhythm rather than against it.

Biochemist Noelia Martín explains why the night is key: "The skin follows a circadian rhythm. At night, metabolic activity in the skin increases, and processes like cell renewal and the synthesis of the protective barrier are boosted." In other words - it's precisely while you sleep that the skin is busiest repairing itself, and the mask helps it along.

The ingredients are familiar from serious skincare: hydrolysed collagen, hyaluronic acid, niacinamide, peptides, ceramides, and some even contain sleep-supporting ingredients like melatonin and magnesium. The masks come in several forms - hydrogel sheets, creams and gel-creams - but the logic is the same: prolonged contact and prolonged hydration.

The result in the morning is visible: a more even tone, better texture, firmer skin and fewer traces of fatigue. Martín adds that the masks are most useful for dehydrated, tired or mature skin, as well as after travel or a change of season - exactly when the skin is under stress.

And there's one warning worth remembering: longer doesn't mean better. Don't combine an overnight mask with aggressive active ingredients on the same evening, and don't think that leaving it on for two days will double the effect - it'll just give you irritation. As with many things, here too less but done right beats more but done blindly.