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Clouds hover above the world's highest peak Mount Everest, left, and Mount Lhotse, right.

Binod Joshi/AP

About 4,000 metres above sea level, you don't sleep as well. You constantly have a headache, or you're taking a lot of Aspirin and drinking a lot of water to fight it. You're sluggish. Your brain is definitely a bit fuzzy. It's almost like being hungover, but you're not actually hungover.

The weird thing about altitude is it affects everyone differently. You get people who don't really report any symptoms at all, and then other people say they feel like they're on the edge of death. It's really rough on them.

A friend at the University of British Columbia's Okanagan campus and I were asked to go along on a trip to Nepal earlier this year to do some research on brain function with an international team looking at altitude sickness. My academic reputation is that I do a lot of work with portable EEG (electroencephalogram) systems, using methods to measure EEG in the field, not in the laboratory. The idea was to collect brainwave data to assess brain function with regard to learning and general cognitive function and decision-making at a bunch of different altitude points, all the way up to Everest Base Camp, which is more than 5,300 metres high.

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Our subjects were members of the research team itself, so we collected data on 22 people at five different elevations. We're still working on the data, but there's no doubt that brain function and decision-making systems are impaired with altitude.

Much of the Base Camp trek is quite well developed, especially the first half. It's like stone staircases, and while we were walking along, we would all chant, "We hate stairs. We hate stairs," because they're so big and steep.

You're only supposed to go 400 metres up in a given day, so the physical distances aren't that long – you might only walk seven or eight kilometres in a day – but you're gaining enough altitude that you have to spend the night when you stop. Only one person in our team got altitude sickness, and that was only on the last day. We attributed that largely to the fact we were doing it the way you're supposed to be doing it – we avoided alcohol, we were eating well and drinking fluids and we had a few acclimatization days, where we wouldn't gain any altitude so we could get used to it.

There's no definitive answer to why altitude sickness happens. The technical term is acute mountain sickness. The pressure of oxygen changes, and your body has trouble dealing with that. Technically, these things start to happen at about 2,400 metres. People report flu-like symptoms: headaches, fatigue, stomach illness, dizziness. If you stay at the same altitude, your body will adapt and those symptoms will go away. The Nepalis don't feel any of this stuff. At one point, our sherpa ran ahead for 45 minutes uphill carrying a pack. We just watched him, thinking, I couldn't run up here if you paid me. It's not possible.

The two conditions people typically die from are high-altitude pulmonary edema or high-altitude cerebral edema, which means bleeding from the heart or the brain. Generally, the rule is if your symptoms get too bad, turn around and go down.

People now think that genetics is a possible explanation for altitude sickness. Physical fitness has almost zero correlation with whether you'll get it or not. The year before, we were trekking in Peru and the guy who went down the hardest was an Iron Man athlete. He was in incredible shape, but one day, he just collapsed, and they eventually had to carry him into camp on horse. People once thought that a bit of body fat might help you, and maybe it does. But there are definitely people carrying extra pounds who get hit with altitude sickness.

What we do know is that our EEG data definitely show a drop-off in cognitive function, especially at extreme altitudes around 4,000 metres. It's probably due to a lack of oxygen in the brain, but there may be other factors. If your body isn't functioning normally, your brain's not going to function normally.

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So it's a tough trek because your head's not in the right space. And you're just going up and up and up. I'm in my 40s and my knees were not in love with me. You're just constantly sore. And you're not sleeping well. We were all on the drug Diamox, which you take to alleviate the effects of altitude sickness. It's a diuretic, so you try to get a nice sleep, but you're waking up three to four times a night to go to the bathroom.

At 5,200 metres, everyone was feeling it. No one was sleeping well and a lot of people didn't even remember having dinner that night, even though we were all in a big room eating together. I didn't remember going to the bathroom, even though my roommate told me I'd got up to go twice during the night.

I woke up groggy, but I'd say I had an okay sleep. The year before in Peru, we got up to 5,200 metres and I was violently ill. My biggest symptom was what's called altitude-induced sleep apnea. What happens is you're basically exhausted, you literally pass out and 30 seconds later, you're wide awake choking for oxygen. Then you finally calm yourself down and get your breath back, and you literally fall back and pass out again. So you're going from deep sleep to wide awake and not breathing to breathing, and back to deep sleep again every two to three minutes for hours at at time. It was horrible. I was in tears. The whole time on Everest, I was living in fear that would happen again. So my sleep wasn't too bad relative to that.

After we reached Base Camp, we trekked up Kala Pattar, which is a mountain, almost like a ridge, that's at nearly 5,500 metres. That was the end of our ascent. When we reached the summit, I felt elation. That was a personal best for me. I'd never reached such altitude before.

From there, we watched the sun rise over Mount Everest, and the view was breathtaking. The skies were crystal clear and we had a view of the whole valley. What more could you want?

Olav Krigolson is a neuroscientist and an associate professor at the University of Victoria.

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As told to Wency Leung

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