All of the victims are members of an Antioch family and were traveling east in a 1999 Chevrolet Tahoe at the time of the crash, CHP Sgt. Paul Hurt said.
The driver of the SUV, 20-year-old Estefania Granados, was apparently fatigued and lost control of the vehicle around 12:45 a.m. west of Loveridge Road, Hurt said.
The SUV rolled onto a dirt shoulder near Railroad Avenue, and officers arrived to find it overturned.
Hurt said the reporting officer noted that four of the family members did not appear to have been properly restrained at the time of the crash.
All eight occupants of the car were hospitalized. Granados suffered moderate lacerations, according to the CHP.
Most severely injured in the crash was a 1-year-old who suffered a broken leg and was airlifted from the scene by helicopter. A 2-year-old and two 16-year-olds were also hospitalized for minor injuries.
Passenger Eric Granados, 21, reported back pain; Adan Grandos, 26, complained of head pain; and Monica Sanchez, 21, reported pain to her head, back, and hips, CHP officials said.
Investigators do not believe alcohol or drugs played a role in the crash.
The CHP is asking anyone with information on the crash to call its Contra Costa office at (925) 646-4980.