jwddboy said:
I believe all sleep but leave their natural reflexes in control of their body.
Most critters that can be or were prey have sleep phases. They need to be on the alert for danger, so they'll have "hover" cycles (my horses will sleep standing up, appearing completely relaxed, but if there's a huge noise, they bolt immediately; my tetra sort of hover in a group, barely moving). Mammals that sleep in small, protected places like dugouts or caves (wolves, badgers, foxes, gophers, praire dogs, cats,etc.) sack right out--often in a protective pile. Sometimes there will be a lookout, and the pod, school, herd, etc., can get a little deeper, more relaxed, or longer shuteye. Fish seem to have an automatic pilot for hover-sleep, but I've seen my gourami sleep on the tank floor, rising up in the morning. One morning I thought my black ghost knife was dead--he was lying on his side in his cave. Sleeping!!! Read up and found they're notorious for this. I'm sure other folks here have other experiences/opinions, but this is what I've observed.