I'd read about the Thundershirt but I'm cheap and skeptical so I waited three weeks before the lack of sleep broke me down and I finally ordered one. It is a game changer for us, guys. We followed the instructions carefully, introducing Circe to it with a short wear and a few treats. She seemed more bemused than anything. But the first night she wore it she slept like a log. Barely any movement, no growling, no barking. We kept checking to make sure she was still breathing. We've only been using it for a week, but so far it's amazing. We put it on her every night when it's time to go to bed and if we're going to leave her alone for a few hours during the day we'll put her down for a nap with it. She's still adjusting to it a little bit, and she seems uncertain about walking much in it (maybe because we only put her in it for bed?) but she doesn't mind it.
I guess it's equivalent to swaddling a baby? Side bonus - I find it really fun to "unwrap" her in the morning. She completely comatose in the shirt, then she wakes up and perks up immediately once you take it off.
I researched it obsessively before purchasing, so I can tell you that it seems to work for some dogs and not others. However, since they offer a money back guarantee (as long as you don't get it embroidered!) it's definitely worth a shot. Circe is 22 pounds of compact muscle and the medium fits her well. You can order them directly from the manufacturer or on Amazon and I think some pet stores carry them as well.
It seems silly to have to say this, because I've never done a sponsored post, but I purchased the Thundershirt on my own and the company has no idea who I am.