GMC 2006 Yukon XL Owner's Manual page 13

CAUTION:
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Sitting in a reclined position when your vehicle
is in motion can be dangerous. Even if you
buckle up, your safety belts cannot do their
job when you are reclined like this.
The shoulder belt cannot do its job. In a crash,
you could go into it, receiving neck or other
injuries.
The lap belt cannot do its job either. In a crash
the belt could go up over your abdomen. The
belt forces would be there, not at your pelvic
bones. This could cause serious internal
injuries.
For proper protection when the vehicle is in
motion, have the seatback upright. Then sit
well back in the seat and wear your safety belt
properly.
Do not have a seatback reclined if your vehicle is moving.
Head Restraints
Adjust your head restraint so that the top of the restraint
is closest to the top of your head. This position reduces
the chance of a neck injury in a crash.
Pull the head restraint up to raise it. Push the head
restraint down to lower it.
Your rear seats may have head restraints that can be
adjusted up and down.
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