The holidays are upon us, which means a huge chunk of America — about 48.7 million people — is going to be flying somewhere to make it to Thanksgiving dinner on time. That also means a ton of people plonking over and snoozing at 39,000 feet in the air.

But if you’re like me, you’re passed out way before the plane you’re sitting in makes it that high. I’ve been awake for takeoff maybe three times in my entire life. Most of the time, I’m fast asleep by the time takeoff happens. I don’t sleep well unless I’m lying down on land, but during the flight, I’m powerless to keep myself from nodding off.

This makes me, for all intensive purposes, a narcoleptic on planes. So that begs the big question: What makes people fall asleep on planes so easily?

The answer is multifaceted. One big factor is cabin pressure. Airlines are required to keep the cabin pressure at the equivalent of 6,000 to 8,000 feet in elevation. (For reference, Denver is about 5,280 feet above sea level.) After 7,000 feet, the saturation of oxygen into hemoglobin in the blood drops. This is critical — it means the body takes up less oxygen with every breath. Less oxygen, of course, means the body’s performance will take a hit. If you’re not already acclimated to this type of environment, you’ll start to feel slower and more tired, as the body begins to spin things down in an effort to reduce energy expenditures to compensate for a lack of oxygen. Before you know it, your breaths get slower, shallower, and conk!

Cabin conditions tend to create drier air as well. Humidity levels are typically lower, and the dry ambiance can make you feel listless and inert. Combine that with having to stay seated for an extended period, and you’ll start to feel lethargic and sleepy pretty fast.

There’s also a paradoxically physical reason you’ll get tired on planes, especially during takeoff — acceleration. When the aircraft is speeding down the runway and about to take flight, you get pushed back. Accelerative forces push the body back, tricking the brain into the surreal sensation that you’re horizontal.

The number one reason why you’re likely to hit the hay as the plane taxis the runway? Ambiance. Aircraft make for a pretty optimal setting for sedentary activity: The lights are dim, the seats are cushioned, alcohol or other preferred beverages are available, blankets and pillows are abundantly passed out, and the whooshing sound of the plane in flight offers a kind of droning white noise that quiets the mind. In short, a flight is already fitted with the prime ingredients for facilitating a nap. All it needs is an extra boost — in this case, a small decrease in oxygen intake, and a bit of force that helps you feel like you’re rocking away softly.

Photos via YouTube / Giphy, Getty Images / Peter Macdiarmid

Neel is a science and tech journalist from New York City, reporting on everything from brain-eating amoebas to space lasers used to zap debris out of orbit, for places like Popular Science and WIRED. He's addicted to black coffee, old pinball machines, and terrible dive bars.