V. B Dwaraka

A drop of blood on filter paper preserves not only cells but also a record of what has been happening to the body over the past few months. That record is not in the genes but on them: the chemical marks cells attach to DNA change with food, stress, illness and age. Scientists have learned to read this archive tape to reconstruct a picture of health without repeated trips to the lab.

The hot air of a sauna makes your heart beat faster and your blood vessels widen. But under the skin something less obvious happens: heat leaves marks on the packaging of DNA. These marks are chemical tags that do not change the text of the genes themselves, but they influence which pages get read. For the first time, scientists have checked how regular sauna use rewrites such marks in living people.