Do worms sleep, for example?
Or sponges?
sleep is a period of unconcious activity where the organism recharges its electrolytes
the length of sleep is only long enough to achieve this
could be you are right
some humans need less sleep than others
dont know about the rem though.
“recharges its elctrolytes”??? Um, that would involve drinking Gatorade.
Madhat, I don’t really know the answer to your question, but my guess is that it somewhat depends on your defintion of sleep. For instance, I do know that fish go through periods where they are relatively inactive and exhibit brainwave patterns considered typical of sleep, but they don’t normally close their eyes.
Given a tentative definition of sleep as “relative inactivity, with characteristic brainwave pattern changes”, you obviously can’t determine whether or not very primitve organisms sleep, due to the absence of brainwave activity in the first place. Worms have very simple nervous systems (in the hundreds of neurons) which are mostly sense and control systems - almost nothing in the way of central processing or “brainlike” functions. So, while they may or may not have periods of inactivity, it is impossible to say whether that would be called “sleep” or simply “resting”.
mischievous
It would take a lot of effort to evolve eyelids just before bedtime, perhaps? 
mischievous is right depends on definition:
RE worms:
Many worms species do have periods of inactivity called quiesence. Quiesence is more of a hibernation, where the worm responds to something like low temperature levels. it will wrap itself in a casing (usually of mucous) and goes dormant until conditions improve.
If that doesn’t do it re the OP, I’d vote spiders. Many species can withdraw at regular intervals (nocturnally or diurnally) pull thier legs up to themselves and drop thier metabolic rate. They can overwinter doing the same thing. So like a bear, they sleep once a day and hibernate (differs from worms in that AFAIK worms don’t do this daily/nightly - just “hibernate” when things go bad)
mrcrow, you give some of the oddest answers I have seen in a while. Please remember that the SDMB is here to fight ignorance. Standards in GQ are quite high. If you state something as a fact, be prepared to back it up with valid cites from reputable sources.
BTW, electrolyte replenishment is a process mostly unrelated to sleep.
QtM, MD (who has been required to correct critical electrolyte imbalances in hundreds of patients)
Well, I was really thinking about regular sleep, rather than hibernation.
What got me started was an article in Scientific American where a sleep researcher said he was unconvinced of any biological “reason” for dreaming (such as to sort out the day’s events, explore phobias, or to clear out useless info … things like that which have been proposed). Instead, he saw dreaming as a simple “side effect” or by-product of two other trends - increasing mental capacity, and the idleness of sleep. Thus the brain had more time than real business to attend to and was simply idling until needed again - that going without dreams would not affect the quality of sleep.
So, that raises the question of when sleep began, and whether there are primitive animals that sleep but do not dream.
Ants sleep for about 3 hours or so every night. They even stretch and yawn when they wake up before they start work.