Do jellyfish sleep?

I like “quoughts” (questions that provoke thoughts) and “Do jellyfish sleep?” is a good one. Jellyfish are interesting for lots of reasons, one of them being their place on the evolutionary landscape. The first jellyfish-like creatures (the ancestors of modern jellyfish) pitched up during the Cambrian explosion, more than 500 million years ago and are one of the first animal groups to boast organised tissues and a nervous system. There is nothing that could be described as a brain, but jellyfish neurons work in exactly the same way as vertebrate neurons. So do jellyfish experience anything akin to vertebrate sleep?