It's been proven in studies that a deer, head down, can duck an arrow faster than a head up deer.
When a head down deer hears the bow, as it jerks its head, that motion forces its body down, resulting in the kill zone dropping.
With a head up deer, it has to lower its head to bend the legs, resulting in a microsecond where the vitals stay in the kill zone.
Can't disagree with that.
Dr. Woods explains it very well.
What I should have said was a relaxed or distracted deer, as opposed to an alert one staring you down.
Another issue is wind & other sounds which can muffle your bow noise.
Let me rephrase this a bit . . . . .
And it's often not the sound they react but instead they actually see the sudden movement of the bow limbs. Deer commonly "jump" the string. And they can at 40 yds with a fast crossbow if they "see" those bow limbs quickly twitch.
That said, in the right circumstances, such as a feeding deer, head down or head up, a little beyond 35 yds can still be high probability, unless it just suddenly decides to take a step as you release. Then you have a gut shot deer.
My preference is for the deer to have it's head up, and looking the other way, as a part of it's routine "checking" rather than being "alerted" by something.
Dr. Woods made the case for always aiming at that bottom 1/3.
I've always aimed for the bottom 1/4, in part to likely still hit in the vitals if the deer drops (which they may or may not), and in part because a heart shot (or low shot) more often provides a quicker, better trail (and animal recovery) than a high shot.
Ideally, I like a steady breeze, rattling the leaves, and then have a shot at a relaxed deer, broadside, head up if not looking towards me. On feeding deer, it is sometimes possible to take advantage of opportune noise to release your arrow, such as when an airplane or helicopter's sounds peak, or when there is a wind gust peaking.
Another shot I like is with the deer actually steadily walking, but certainly under 25 yds.
While some say do something to "stop" the deer, I usually prefer to just take the shot,
with less risk of the deer "jumping".
Your mileage may vary.