Sleep is as old as time, a new study found.<br /><br />Humans and other mammals need sleep but there has been debate whether all animals need it and why animals sleep. <br /><br />Theories involve everything from memory to learning to cellular recovery.<br /><br />Now scientists found a primitive brainless and spineless jellyfish called Cassiopea which spends its entire life sitting upside down on the ocean floor sleeps too.
