It’s important that the headrest in your car, also known as a head restraint, does not encourage or force you into a head forward posture.
It’s there to prevent whiplash, not to make a whiplash event more likely or harmful. Which it can, if it’s not adjusted properly. Pushing your head into a more forward position is not just a nuisance or discomfort to put up with.
If your headrest is positioned close to, or lightly touching the back of your head when you’re sitting in your most-upright-yet-relatively-comfortable-and-at-ease position, then you’re fine. Otherwise, read on.
What the National Highway Traffic Safety Administration says
Hacks for adjusting the headrest when your car seat is not cooperating
Why is this so important:

When the head is forward of the body, anything deviating from what you see here, your head becomes much heavier to hold upright. Based on simple physics, for every inch forward your head is of the rest of your body, you can figure about another 10 pounds of work for your neck muscles to hold your head upright. But the neck muscles can’t do all the work. Something else gets recruited – the joints between head and tailbone.
Another problematic consequence of head forward posture; You lose what’s called the lordotic curve, part of the double S shape curvature of your spine that allows your spine to act as a shock absorber. With the lost of that lordotic curve, your neck loses vertical height. Actually, your entire spine shortens. What commonly gets blamed on age but really does not have to happen. The shorter neck, by the way, is not just what’s in back. It’s also, of course, in front, underneath your jaw. That’s when skin wrinkles in the neck inevitably become more prominent.
How does this show up over time as you become used to it? Osteoarthritis along the joints of your spine. The cervical (i.e., neck) spine in particular. Osteoarthritis is the process your body uses to slowly fuse the joints – the joints in your neck, in this case. Results? Less range of motion, more stiffness and likelihood of stress on nerves, and – here’s the key point – less flexibility.
Less flexibility for withstanding the impact of being rear-ended, or any whiplash event. In a healthy spine, the muscles and joints throughout every segment work as a synchronous team, in which all segments immediately adjust to any one segment being even slightly compromised or going off balance – from microscopic adjustments to sudden impacts. Otherwise, you wouldn’t be able to walk much less play sports. Comparatively, in an osteoarthritic spine, a whiplash event can be likened to the force of breaking a brittle stick.
Hacks for adjusting the headrest when your car seat is not cooperating