When hummingbirds sleep, they go into a hibernation-like state called Torpor (pronounces TOR-per). This is a really deep sleep. Their metabolism will lower to one-fifteenth (1/15) of normal. Their body temperature will drop to the point of becoming hypothermic. Their heart rate will drop to about 50 beats per minute. Their breathing will slow to the point that it looks like they have stopped breathing. By sleeping like this, hummingbirds can save up to 60% of their available energy.
A hummingbird will settle in a favorite perching place that they feel safe in. If the hummingbird is a female with a nest of baby hummingbirds that cannot care for themselves, the mother hummingbird will sit on the nest. They will settle in with their neck retracted and their head forward. Their beak will point up at a sharp angle and their feathers will fluff out, making them look like a cotton ball.
When hummingbirds sleep and are in the Torpor state, they have been known to hang upside-down. If you find a hummingbird that is hanging upside-down and they appear to be dead, it is actually more likely that they are just asleep. They will probably not even respond if you touched them. If at all possible, leave them alone and they will wake up when they get warmer. If you think the hummingbird is in need of medical or emergency care, check out the First-Aid section of this website before doing anything.
It takes anywhere from 20 minutes to an hour for a hummingbird to fully recover from torpor. Once they are up and about, the first order of business is food. The hummingbirds will eat 25% of their daily intake as soon as they recover from torpor.
Hummingbirds that are very weak have been known not to survive Torpor. And while this is very sad, it is a part of the natural process. However, hummingbirds need to sleep, just like the rest of us and by going into Torpor, their bodies are just that much more efficient.