Mama Britney is moving up -- after months of a carefully constructed comeback while under conservatorship, she enjoyed her first overnight visit with sons Sean Preston, 2, and Jayden James, 1. It's the first overnight visit since her involuntary psychiatric hospitalization in January, reports Us Weekly.

"Britney was very upset after the kids left," a Spears source says. "She now does not know when she will get them next for an overnight. She complains it could be next week, or it could be three months."

The 26-year-old pop singer -- whose life is still under the control of father Jamie -- was granted overnight visits last week. She and ex-hubby Kevin Federline are due back in court August 25 to continue their fight over custody arrangements -- which they have yet to agree on outside of court. K-Fed wants sole physical and legal custody, with Britney continuing on current visitation terms.

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