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It was a small room, at the top of the house. For a time, it was home to tropical fish. Later, two pet mice slept there, in a home made of fruit crates. The walls of the room were covered with posters of Madonna and Duran Duran. Then it was the Rolling Stones. Then Jimi Hendrix.

This was the childhood bedroom in Oxford, England, of James Mollison, 37, a documentary photographer who was born in Kenya and now lives in Venice. He had the luxury as a boy of adapting his bedroom to reflect his changing interests.

“As a child, that’s your little space within the house,” Mr. Mollison said.

James Mollison Kaya, 4, lives with her parents in a small Tokyo apartment.

James Mollison Kaya’s bedroom.

Mr. Mollison’s new book, “Where Children Sleep,” had its origins in a project undertaken for a children’s charity several years ago. As he considered how to represent needy children around the world, he wanted to avoid the common devices: pleading eyes, toothless smiles. When he visualized his own childhood, he realized that his bedroom said a lot about what sort of life he led. So he set out to find others.

His subjects came from Boy Scout troops and sumo wrestling clubs. They were introduced through friends of friends. Mr. Mollison posed his young subjects — more than 200 of them — in front of blank white backgrounds for their portraits, leaving their bedrooms to do the talking. More than 50 pairings are in the book, which has a glow-in-the-dark cover (a nod to the glow-in-the-dark stars on so many childhood ceilings).

As much as the project is about the quirkiness of childhood, it is, more strikingly, a commentary on class and on poverty. But the diversity also provides a sense of togetherness.

Everybody sleeps. And eventually, everybody grows up.