Demi Lovato OD'd After All-Night Party

Demi Lovato's overdose came at the end of a party at her home that lasted all night, but when EMTs and police arrived to help her ... the partygoers were nowhere in sight ... law enforcement sources tell TMZ.

Our sources say when first responders arrived at Demi's Hollywood Hills home Tuesday just before noon, they were told there had been a party. We're told members of Demi's team were present when first responders arrived, but provided no information about the drug or drugs that caused her to OD.

It's unclear when the party broke up, or if people scattered once it became apparent Demi was in trouble.

TMZ broke the story ... the night before the emergency, Demi was at the Saddle Ranch restaurant/bar celebrating the birthday of one of her backup dancers and she seemed happy and lucid. We don't know if the people who were at her home were part of the group that partied at Saddle Ranch.