Hummingbirds have a metabolic rate that is 77 times that of the average human. Because of this, they must feed continuously. Unfortunately this is an issue when night rolls around. It becomes too cold or dark for their to keep feeding. Well, they have a defense mechanism for that. They lower their body temperature by 10˚C to 30˚C. This will slow their metabolism by 95% which protects them from starvation.
Apparently the birds become motionless at this time. You wouldn't even be able to tell if they are alive!