From midnight until 4 a.m., inbound passengers headed downtown from O'Hare will have to take a shuttle bus from the airport to the Rosemont stop where they can catch blue line trains.
The CTA says this change allows additional cleaning of trains at the O'Hare station and will be in effect until further notice.
Outbound service into the airport will not be affected, CTA officials say.
For more information on train schedule changes, visit the CTA website.