"Which is more difficult for humans to adapt to: 55°C above zero or 55°C below zero?

55°C above zero overlooks an important condition—whether it's dry. If the air is sufficiently dry and with adequate replenishment of lightly salted water, humans can survive normally by relying on sweat for cooling. Humans possess the most exceptional sweat-based cooling ability among mammals; no other mammal can sweat to the point of becoming drenched like humans.

The heat carried away by water evaporation is substantial, and air's thermal conductivity is relatively low, making an 18°C temperature difference manageable. For reference, consider dry saunas, where temperatures range from 80–85°C, sometimes even higher, up to 120°C. In contrast, steam rooms (wet saunas) typically stay at 45–60°C and rarely go higher, while Turkish baths at 100% humidity dare not exceed around 40°C because sweat-based cooling fails completely. In a dry 55°C environment, you might not even feel like you're in a sauna—those who have the chance can try it or experience the vast difference in perceived temperature due to humidity in dry regions like the northwestern parts of China.

This is because, in dry conditions, air's thermal conductivity decreases further, while convection actually enhances sweat evaporation efficiency.

Personally, I’ve slept while sweating—once you manage to fall asleep the first night, you can sleep normally afterward.
Moreover, as long as evaporation is fast enough, that sticky feeling diminishes.
Thus, the order of difficulty in adaptation should be: humid 55°C above zero, 55°C below zero, and dry 55°C above zero.
There is no 'humid 55°C below zero' because at -55°C, the air is inevitably dry—water vapor freezes and cannot remain stable."