Most people don’t sleep well at altitude. Climbers commonly report vivid dreams, feelings of being suffocated and wake feeling unrefreshed. Disturbed sleep forms one category of the self-assessment score sheet that is used to diagnose altitude sickness, although this is controversial as other factors might affect sleep at altitude, not just acute mountain sickness. The cold, the wind, noisy or smelly tent companions and long distance travel can all disturb a restful night’s sleep. However, there are important changes in the way we sleep at altitude that makes sleep quality poor.
Sleep can be divided into stages that are defined by the pattern of electrical activity in the brain and eye movement. Deeper stages of sleep and rapid eye movement (REM) sleep are reduced at altitude, therefore more of the night will be spent as light sleep and sleep quality will not be as good as at sea level.
Periodic breathing (Cheyne Stokes breathing, or PB) is common at high altitude and becomes more frequent with increasing altitude. Periodic breathing involves alternating periods of deep breathing and shallow breathing. Typically, three to five deep breaths will be followed by a couple of very shallow breaths or even a complete pause in breathing. A pause in breathing like this usually lasts around 5 to 15 seconds and is called an apnoea. Apnoeas may end with a gasp that sometimes wakes the individual or their sleeping companions! People may breathe this way for most of the night.
During apnoeic phases, oxygen levels drop and heart rate slows. Oxygen levels and heart rate rise again when breathing resumes resulting in cyclical variations in heart rate and the amount of oxygen in the blood.
Low oxygen levels overnight are likely to disturb sleep but PB may also contribute to arousals: periods when you almost or completely wake up. Arousals are more frequent at altitude, but they can occur even in the absence of periodic breathing. Perhaps surprisingly, although PB may disturb sleep, it doesn’t seem to make the other symptoms of acute mountain sickness worse.
At sea level the build up of the waste gas, carbon dioxide, in the blood controls breathing. If you hold your breath, carbon dioxide levels rise and create the urge to breathe. At high altitude, the body senses low oxygen levels and this becomes the main drive to breathe. Breathing faster and deeper at high altitude leads to a profound reduction in the carbon dioxide levels in the blood. You can read more about the effects of breathing harder at altitude here. During sleep at high altitude, the levels of carbon dioxide in the blood can drop very low and this can switch off the drive to breathe. Only after the body senses a further drop in oxygen levels do you start breathing again. During the apnoea carbon dioxide levels rise but levels fall again when ventilation resumes, continuing the cycle.