On Colorado’s Mt. Bancroft, I woke up in the middle of a long winter night, heavy-headed and gasping. My heart was beating like a guy who just ran a marathon at 15,000 feet. A snowstorm had buried the perimeter of our small, two-person single-wall tent and covered the vents, cutting off airflow. But I didn’t realize what was happening—or make a move to fix it—until much later, when my buddy went out to pee and a wave of fresh, oxygenated air rolled in the open door reviving us. We laugh about it now—the most lethal part of our climb happening in our sleep—but I could have died of carbon-dioxide poisoning. Ventilate the tent (open a gap in the door, if needed), shake off fresh snow, and don’t always whiz in a pee bottle—poking your head outside once in a while could save your life.