According to TMZ, the LOL star called 911 late on January 23. Once paramedics arrived at Moore's house, she was evaluated and taken to a Los Angeles hospital.
She has since apparently checked into a separate facility to "seek further professional assistance".
TMZ quotes a source alleging that the hospitalisation was due to a substance abuse problem; Moore's rep puts the move down to "exhaustion".
Moore's spokesperson said: "Because of the stresses in her life right now, Demi has chosen to seek professional assistance to treat her exhaustion and improve her overall health. She looks forward to getting well and is grateful for the support of her family and friends."
The actress, who split from husband Ashton Kutcher in November, recently confessed that she feels unworthy of love since the split.
Moore has apparently signed a £1.3m ($2m) book deal to discuss her divorce from Kutcher in a new autobiography.