The American Museum of Natural History is hosting its first adults-only sleepover this week, with a champagne reception, live jazz and a three-course dinner. "Every museum is looking for a new way to engage different demographics," said Paul Johnson, a fund-raising consultant. "It's about cultivating future audiences and future donors." Scores of museums offer overnights for children, popularized by the children's tales "From the Mixed-Up Files of Mrs. Basil E. Frankweiler" about runaways hiding in the Metropolitan Museum of Art and the book and movie "Night at the Museum," about a watchman who discovers exhibits come to life after visitors leave. Children taking part in overnight programs can solve crimes at the Carnegie Science Center in Pittsburgh, explore caves at the Cincinnati Museum Center, watch sharks at the Aquarium of the Bay in San Francisco and snooze inside a submarine at the Wisconsin Maritime Museum.