National Transportation Safety Board investigator Ted Turpin says Wednesday that the operator woke up when the train jumped off the tracks and hit the escalator early Monday.
He says the woman also admitted she'd fallen asleep on the job in February, partially missing a station.
She has been a train operator for about 60 days.
Monday's crash at O'Hare International Airport's underground station happened around 3 a.m., injuring more than 30 people.
The NTSB is investigating the woman's training, scheduling and disciplinary history.
Crews were cutting apart the lead car of an eight-car train that was damaged during the crash.