Scientists have identified two blood chemicals linked to itchy eczema, offering new treatment possibilities.
Eczema affects one in 10 babies
The Chinese University of Hong Kong devised an "itchometer" that was worn by 24 children and monitored how much they scratched while they slept.
Researchers found that as scratching increased, so did levels of two specific blood chemicals, reported the British Journal of Dermatology.
A British expert said the finding could point to the skin problem's root cause.
One in 10 babies are affected by the dry, scaly, skin rashes of eczema, and the condition can persist into adulthood.
Sufferers can be treated with steroid creams, but the mechanisms behind the "eczema itch" are complex and poorly understood.
The latest research adds to evidence that two specific chemicals found in the blood - "brain-derived neurotrophic factor" (BDNF) and "substance P" - are somehow connected to itchy sensations.
The scientists recruited 24 children, with an average age of 11, to wear a wrist monitor which recorded wrist movement during the night.
The assumption was that this could record when the child scratched in their sleep - a reliable indicator of the level of itchiness.
Blood tests from the children showed that as night-time scratching increased, so did the levels of the two chemicals in the bloodstream.
Kam-lun Ellis Hon, one of the researchers, said: "As far as we are aware, this is the first report to demonstrate that BDNF and substance P are significantly linked to disease activity, quality of life, as well as the levels of scratching as recorded by the wrist monitor."
Dr Colin Holden, the President of the British Association of Dermatologists, welcomed the report.
He said: "For most eczema sufferers, itching is the worst symptom of the disease. It is known to keep children awake at night, which in turn affects parents and can put pressure on the whole family, and even affect children's performance at school.
"It is by discovering the mechanisms behind the disease and its symptoms that we can develop new therapies that specifically target the root cause of the problem."