How do hummingbirds sleep?

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Ooh this is an exciting question! I’ve spent the past 5 years thinking about just this!

Look at these infrared images from a crowd-funded project we did in the summer of 2017: The bird on the left is generating heat, keeping itself warm, while the one on the right is in torpor. It has allowed its body temperature to become the same as the air temperature and has stopped ‘thermoregulating’ - maintaining a high body temperature.

Hummingbirds find a nice and sheltered place at night, and they latch on to a branch with their tiny feet, and then they go to sleep. Some of them, as others have said here, use a strategy called torpor, where they can lower the amount of energy they use by ~85%. They do this by basically shutting down a bunch of their bodily functions- they allow their body to get cold as the night gets colder. You and I spend a lot of energy keeping our bodies warm so everything functions normally. Hummingbirds in torpor give up this ‘normal’ function, and become more like lizards, in that they can get ‘cold-blooded’ in torpor. Torpor is a tricky state to be in, because they can’t respond to outside stimuli for 20–30 minutes, until they warm their bodies back up. They take that risk just to have enough energy in their tiny bodies to make it to the next morning!