As scary as it sounds, it’s common for people to repeatedly wake up gasping for air as if suffocating when sleeping at high altitude. Although there’s a lot of variation in how people respond to altitude, most people suffer from sleep disturbances to some degree. Problems often become noticeable at around 3,000m and upwards.

For some, the feeling is like near drowning. It could be compared to diving too deep under water then, having desperately surged upward to break through to the surface, taking in air with a panicked sensation of an intensity that can only come from feeling incapable of fulfilling an urgent primordial drive. Unfortunately the experience is more pronounced when drifting off to sleep. It can prevent people from settling into stable sleep for hours into the night.

A High Altitude Sleep Study

I wanted to share some of my personal sleep study data here, as I think it really sheds some light on what sleep at high altitude feels like for many people. The data comes from a research project conducted on high altitude physiology, in which I participated as both scientist and subject. It was collected on an expedition to a high altitude laboratory known as The Pyramid, which is at around 5,050m, near Mt Everest in Nepal.

You can see in the images above that, whilst I’m breathing smoothly and rhythmically at sea level, the situation at altitude is strikingly different. At altitude there is an obvious and abrupt stop-start pattern, known as periodic breathing. The apnoeas, marked in purple, are periods in which I had literally stopped breathing. Unsurprisingly, these periods are associated with marked drops in oxygen. In fact, my oxygen levels at altitude only ever return to a high point of 75%, compared with the normal 97% at sea level.

On average, I had a whopping 122 of these apnoeas per hour, which means I stopped breathing more than twice per minute over the entire duration of the night. I also had an average of 77 arousals per hour, which is more than once per minute over the whole night. Although these arousals may not have always equated to a full conscious awakening, they certainly limited my ability to establish stable and restorative sleep.

This is a fairly typical situation and it can mean waking up in the morning feeling like you have a really bad hangover. As a climber, this is the last thing you want. The daytime demands of climbing a big mountain are usually plenty enough to deal with in themselves.

Physiology of Sleep at High Altitude

A lot of people don’t realise that, under normal circumstances, we breathe in response to increases of carbon dioxide in our system, more so than to decreases in oxygen. Breathing serves two distinct physiological needs. The first is to supply oxygen to the body to facilitate the production of energy, and the second is to remove the carbon dioxide that is a by-product of that energy production. For this reason, the body has receptors that respond both to oxygen and carbon dioxide levels.

At high altitude the air is so thin that a normal amount of breathing doesn’t supply enough oxygen. Consequently, the body’s response to oxygen takes over and increases the amount we breathe. The side effect of this is to drive carbon dioxide down to low levels.

In this low oxygen environment the brain becomes more sensitive to changes in carbon dioxide levels. A small increase in carbon dioxide can elicit a vigorous increase in breathing and, conversely, a small decrease can greatly reduce breathing. There also only needs to be a small drop in carbon dioxide to reach a threshold at which the brain puts a stop to breathing altogether.

And this is exactly what does happen whilst you are asleep – the high sensitivity results in oscillation between stimulation and inhibition of breathing. When carbon dioxide gets low enough, you stop breathing!

Of course, when you stop breathing oxygen levels again plummet and carbon dioxide levels rise. When these get to a critical threshold, the body’s response is to breathe again, vigorously. It’s at this point that a person might wake up feeling as though they are suffocating. This vigorous breathing drives carbon dioxide levels down again, and so the cycle continues.

Basically, the extreme environmental conditions at high altitude throw our systems out of balance. The instability presents as an alternating pattern of breathing and not breathing that is the result of bouncing back and forth between physiological responses.

Treatment for Sleep Disturbances

There is an old climbing adage, ‘Climb high, sleep low’. As with other aspects of altitude sickness, the most effective treatment for sleep disturbances is to descend to lower altitude.

In terms of medications, Acetazolamide (brand name Diamox) is the drug of choice. It works by stimulating the kidneys to increase the blood’s acidity, which, for the brain, replicates higher carbon dioxide levels and promotes breathing. Studies have shown Acetazolamide successfully reduces periodic breathing, and increases and stabilises oxygen levels. It also decreases the time spent awake overnight, and improves the sense of sleep quality.

The mountains provide a highly demanding environment against which you can really test your limits. High altitude adds some additional hardships, as well as some unique dangers. But these shouldn't stop you from getting out there and enjoying the beauty of these places, as well as the benefits that come from facing their many challenges.

This post has been adapted from a lecture that Andrew presented recently at the University of Melbourne.