At first glance, it looks like a skin condition — but it’s actually a side effect most people never expect. The strange, blotchy pattern on the leg above is caused by sleeping too close to a heater or electric blanket for several nights in a row.

Doctors say this condition, known as erythema ab igne (Latin for “redness from fire”), develops when the skin is repeatedly exposed to low levels of heat for long periods. The warmth doesn’t burn, but it damages small blood vessels beneath the skin, leading to a net-like pattern that can last for months — or even become permanent.

Many people unknowingly cause this by falling asleep with space heaters, heating pads, or electric blankets pressed close to their skin. What feels like comforting warmth slowly causes tiny changes in the blood flow and pigmentation of the skin.

Experts warn that constant exposure can lead to chronic inflammation, pain, and in rare cases, even pre-cancerous skin changes.

So if you rely on heaters at night, always keep them at least half a meter away from your body — and never sleep directly under their warmth. The cozy feeling isn’t worth the marks it can leave behind.