A couple of things - first don't rely on a single detector, two, at different locations is far better, As others have said, the one in the bedroom should be just above pillow height, close to the bed, not in the pat of draughts which would probably cause false readings.
Second, carbon monoxide will clear from the blood over a few hours, so did you feel better after a day at work? (Second thoughts, that's a silly question - who feels better after a day at work...)
The toxicity levels for CO cover a wide band, depending on the level, duration and a myriad of other factors Wiki bits