How to make <li> jump up and down using JQuery?

I have a horizontal navigation menu with a "JOIN" item. I want the #jump item to jump up and down indefinitely soon after the page loads.

This gives the effect but only once and only upon hover:

 <script type="text/javascript">
   $('#jump').hover(function(){
   $(this).animate({bottom:'5px'},'fast');
   }, function() {
   $(this).animate({bottom:'0px'},'fast');
   });
</script>

if I make it: ready() it stops working.