After a three-hour standoff with police last night at her LA home, beleaguered Britney Spears was taken to the Cedars-Sinai Medical Center on a gurney for an evaluation. Police were called around 8 p.m. to Brit's house last night "about a custodial dispute regarding Britney Spears' children," said LAPD officer Jason Lee, after Britney refused to hand over her sons, Sean Preston and Jayden, to her ex-husband Kevin Federline. "While officers were on the scene, they observed Ms. Spears under the influence of an unknown substance," added Lee. Britney was "hysterical and just wanted to keep the kids overnight," an insider told the New York Post. According to the paper, the singer is currently on 72-hour hold at the hospital, where she is undergoing a psychiatric evaluation and is being tested for drugs. Britney is reportedly under suicide watch.