Once the sun is beginning to set, hummingbirds increase their feeding in order to prepare themselves for their nighttime sleep.  They go into a deep sleep similar to a bear going into hibernation for the winter, but they do it every night.  It is called torpor (pronounced TOR-per) and it's a state where the bird slows down all of its body functions for the night.

The metabolism of a hummingbird is extremely high and without constant feeding it cannot survive.  At their daytime metabolic rate hummingbirds would never survive the night, so they slow down their heart rate and breathing rate to a fraction of their daytime rates.  Their body temperature decreases often to the ambient temperature (unless they winter in an area with colder temperatures).  This ability makes a hummingbird a "heterotherm," meaning they can switch between being both warm-blooded and cold-blooded.

If you see a hummingbird in torpor you would probably think it is dead.  Sometimes they go into torpor while sitting on a feeder and you can find them hanging upside down.  You might find one hanging upside down from a tree branch.  If you find a bird like this, just leave it alone.  It is not dead and they won't respond to you even if you pick it up.