The notion that children grow taller while they sleep is probably true, US researchers say.
90% of the growth occurred when the animals were resting
They put sensors into the leg bones of baby lambs and confirmed that most growth spurts occurred when the animals were at rest or sleeping.
The University of Wisconsin scientists believe the same is true in humans, and say it might explain why children get growing pains at night-time.
Their findings appear in the Journal of Pediatric Orthopedics.
Bone length was continuously measured by the sensors every 167 seconds for around three weeks.
At least 90% of the bone growth occurred when the lambs were asleep or resting.
Researcher Dr Norman Wilsman said: "What was really interesting was that the bones were growing only when the animals were lying down, and almost no growth occurs when the lambs are standing or moving around."
The researchers believe that when the animal is at rest, pressure on the bones involved with growth - the growth plates - is eased, allowing them to elongate.
"Growth plates may be like springs that, during standing and walking, experience compression and tension," said Dr Wilsman.
"When these strains are eased, as when the animal lies down or goes to sleep, they resume growing," he said.
Co-researcher Dr Kenneth Noonan said: "Growth is not a continuum.
"There are growth spurts, which may occur within the daily life of lambs and possibly humans too."
Dr Jeremy Wales, a consultant paediatrician who has studied child growth at Sheffield Children's Hospital, said: "There have been human studies that also document this.
"Children do have growth spurts at night."
He said French folk law back in the 1500s claimed children grew at night.
Dr Tom Hutchison, consultant paediatrician in Bath, said it was unclear whether this pattern of growth would be linked with growing pains for a number of reasons.
"The most rapid growth occurs in teens, but growing pains are most common among children aged six to 10."
Children can also get growing pains during the day.
"The important thing is that these pains are a normal part of childhood and patients shouldn't be overly alarmed," he said.