To be on cruise control means you are detached from the world: you glide through, paying limited or no attention to events which occur around you. You often stare into space. You may daydream, but most of the time you aren't really thinking about anything. You may feel lost even in familiar surroundings.
This can result from the break-up of a relationship, trouble within the family or the death of someone close to you.
It may involve drug use.
The term derives from the cruise control system on a car, which allows a specific speed to be set and regulated automatically by an onboard computer; the driver does not have to use the accelerator pedal
Ever since my brother killed himself, I've been totally out of it: I'm on cruise control.