A mattress that's a few years old has absorbed a lot: sweat, skin cells, and whatever settles out of the air overnight. None of that is visible, which is part of why it's easy to forget about.

Dust mites feed on skin cells, not dust itself

The average person sheds a surprising amount of skin overnight, and dust mites feed on exactly that. Their waste particles — not the mites themselves — are what triggers allergy symptoms in a lot of people, which is why an "invisible" mattress can still cause morning congestion or itchy eyes.

Moisture makes everything worse

Overnight sweat doesn't fully evaporate from a mattress the way it does from sheets that get washed. Over time this creates a slightly humid environment inside the mattress that mites and mold both prefer.

Sheets alone don't reach the problem

Washing sheets weekly is a great habit, but it only ever touches the top layer. Everything described above happens underneath, which is exactly what a mattress protector and periodic deep cleaning are meant to address.

If mornings consistently start with a stuffy nose that clears up soon after getting up, the mattress is worth a closer look before assuming it's seasonal allergies.