Rhonda James was skeptical when her husband ordered a weighted blanket for her online. But five minutes after wrapping it around her body, she passed out.

“It was like a really big hug,” said James, a banker from Charleston, in the US state of South Carolina.

But research on the effectiveness of weighted blankets is limited. Here’s what you need to know.

What is a weighted blanket?

Weighted blankets have extra weight in the form of glass beads, pellets, cotton, or other filling. Scientists haven’t studied exactly how the blankets work, but they have some ideas.