Simple, right? Butterflies are active during the day, so at night they find a hiding place and go to sleep. In the same way, moths are active at night and during the day moths hide and rest.
Animals that sleep during the night, like most butterflies, are diurnal. Animals that sleep during the day, like most moths, are nocturnal.
Never seen a sleeping butterfly? A sleeping butterfly would make an easy meal for a nocturnal predator! If you’re dedicated to finding one, check under leaves, in between rocks, or even between blades of grass. Or maybe, because most butterflies only live for a month or two, you should just leave them be: they only get a few dozen sleeps!
If only it were that simple! I would have had less to write and you would have had less to read.
“Most?” I can hear you saying, “Why do you keep saying ‘most’? Aren’t moths the ones that fly at night and butterflies the ones that sleep at night?”
And you would be correct, most of the time.
The reality is, nature is incredibly diverse. There are over 17,500 species of butterfly and 160,000 species of moth. Ten percent of all known species of organisms are either butterflies or moths! Biologists can create simple rules, like “butterflies fly during the day”, but these rules are bound to have exceptions because of the sheer number of species involved. As an experiment, what simple rules could you create to define what separates every single cat from every single dog? Size? Ear shape? Tail length? It’s not so easy!
The Long(er) Answer:
It wouldn’t be right to give so many “mosts” without giving examples. Let’s get to it.
There is a family of butterflies called Hedylidae known as the “American moth-butterflies” that sleeps during the day and is active at night. There is a genus of moth, Hemaris, that resembles bumblebees or hummingbirds. These moths are active during the day and sleep at night.
Hummingbird? No hummingbird I’ve ever seen has had antennae. This is a hummingbird hawk moth!
Most butterflies emerge from a chrysalis. Not so for the family of butterflies called Parnassius, which emerge from a loose, silk cocoon. Meanwhile, the tropical hawk moths of the family Sphingidae don’t emerge from a cocoon at all: their pupas are unwrapped and exposed and the adult moths emerge from underground. How about a moth that wraps itself like a butterfly? Nature has those too: the gypsy moth, an invasive species to the northeast United States, has a pupal stage that looks much more like a chrysalis than a cocoon.
Cocoon? Chrysalis? Tough to tell, but it’s definitely not hatching a butterfly: this is the pupa of the gypsy moth.
Most butterflies only live as adult butterflies for a few months. This is especially true for tropical butterflies like the ones showcased in the Judy Istock Butterfly Haven. Butterfly species that must survive a cold winter, like the Illinois-native monarch butterfly, have an adult phase that lives long enough to migrate south to Mexico in the fall and return home to the central United States in the spring.
So. Where do butterflies go at night? To sleep. But most biologists will give you a much longer answer that most people agree goes a little too far.
By Kyle Schiber, Nature Museum Volunteer
 North American Butterfly Association. (2016) Butterfly Questions and Answers. Retrieved December 18, 2016, from http://www.naba.org/qanda.html
 Smithsonian Institution. (2016) Bug Info: Moths. Retrieved December 19, 2016, from https://www.si.edu/Encyclopedia_SI/nmnh/buginfo/moths.htm
 Mallet, Jim. (19, Jan 2014) The Lepidoptera Taxome Project Draft Proposals and Information. Retrieved January 2, 2017 from http://www.ucl.ac.uk/taxome/
 Scoble, Malcolm J. and Aiello, Annette. (1990) Moth-like butterflies (Hedylidae: Lepidoptera): a summary, with comments on the egg. Retrieved December 18, 2016 from http://www.stri.si.edu/sites/publications/PDFs/Aiello_Scoble%20&%20Aiello%20.pdf
 Taraglia, Elena. (15, July 2015) Year of the Sphingidae – Diurnal Moths. Retrieved December 20, 2016 from http://nationalmothweek.org/2015/07/15/year-of-the-sphingidae-diurnal-moths/
 https://commons.wikimedia.org/wiki/File%3AIC_Macroglossum_stellatarum1.JPG, By IronChris (Own work) (http://creativecommons.org/licenses/by-sa/2.5-2.0-1.0)], via Wikimedia Commons
 Butterflies and Moths of North America. (2016) Attributes of Parnassius Clodius. Retrieved December 20, 2016 from http://www.butterfliesandmoths.org/species/Parnassius-clodius
 https://commons.wikimedia.org/wiki/File%3ALymantria_dispar_-_growth_A_-_07_-_chrysalis_(2009-06-25).jpg, by ©entomart [Attribution], via Wikimedia Commons
 Monarch Joint Venture. (2016) Monarch Migration. Retrieved January 2, 2017 from http://monarchjointventure.org/monarch-biology/monarch-migration/