"She is focusing on getting healthy," a source told the entertainment news outlet.
"She lives in a small room, has a chef that makes her sushi and a fresh garden is available," the insider added. "She is also getting along great with the other residents."
E! said the facility doesn't allow family or friends to visit patients, so Lohan's birthday Tuesday was a low-key affair.
Lohan was court-ordered to spend 90 days in rehab as part of her conviction for reckless driving and lying to police.
She checked into the Betty Ford Center May 3 but reportedly was unhappy with her treatment there and requested a transfer, which was approved last month.
The "Mean Girls" and "Freaky Friday" star's once-bright career has been eclipsed in recent years by her battles with drug and alcohol addiction, motor-vehicle offenses and legal woes.
She was most recently seen in a guest role on Charlie Sheen's sitcom, "Anger Management."