Amanda Bynes will be held for a further two weeks to assess her mental health.
The former child star was placed in an involuntary psychiatric hold fter deputies said she started a fire in the driveway of a home in Thousand Oaks, southern California.
She was detained under the 5150 order, meaning she could be held for up to 72 hours for observation, but she will now be spending two more weeks at the Hillmont Psychiatric Center, part of the Ventura County Medical Centre.
Bynes, who rose to fame on the Nickelodeon series All That and The Amanda Show, has become a source of increasing concern over her erratic behaviour in the last few months.
She was reportedly drunk when she started the fire in Thousand Oaks. Prior to that, she made a court appearance in New York earlier this month after allegedly throwing a marijuana bong out of the window of her 36th-floor apartment in Manhattan.
A photo has emerged online apparently of the actress under a white sheet while being wheeled around by a member of the hospital staff.
Her parents, Lynn Organ and Rick Bynes, have supported the judge's decision to keep her at the unit and are reportedly to attend a hearing on Friday to decide whether they will be granted conservatorship of their daughter.
They will have to convince the judge that taking their daughter into their full care is the best thing for her and that they can provide a safe and stable environment.
A conservatorship would give them the authority to make decisions on certain aspects of Amanda's life.