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I am watching a video about a climb of Annapurna. To acclimate the climbers head up from base camp to another camp higher up, stay for a day or two, then return to base camp. They repeat this process a few times, each time going a little higher.

Why the return to base camp? Seems to me that would partially undo the acclimatization they are seeking.

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What they are doing is following the maxim "climb high, sleep low". Going too fast will cause altitude sickness.

Humans have a lower respiration rate while sleeping, which is why they can be okay at higher altitudes while awake and yet need to descend to sleep.

The accepted guideline is not to increase your sleeping altitude by more than 1000 ft (300m) per day.

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    It's not just about the sleeping respiration rate, though this is a very good point, it's also a strategy to drip feed your exposure to give your body time to adjust. So each time you climb you push it a little further, then descend to give your body time to adjust then go a little further again.
    – user2766
    Nov 17, 2016 at 13:20
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    @Liam Two steps forward, one step back.
    – Charlie Brumbaugh
    Nov 18, 2016 at 5:19
  • @Charlie, can you explain how the lower respiration rate is important? I would have thought that would be compensated for by a lower work rate, but I infer from your answer that's not the case.
    – Toby Speight
    Nov 18, 2016 at 15:49
  • @TobySpeight I am not medical person, but I would assume that it is because the lower respiration rate causes you to get less oxygen, which leads to altitude sickness.
    – Charlie Brumbaugh
    Nov 18, 2016 at 16:05
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    Respiration is triggered by CO content in blood. At high altitude, the O2 content can be very low, but the CO not high enough to trigger breathing and you have to consciously remember to breath. When awake, things happen to remind you, when asleep, you stop breathing till you wake up half suffocated. Climb high, sleep low helps (but not fully addressed) this.
    – user5330
    Nov 19, 2016 at 4:15

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