In bash there's a command called ":", which is always evaluated to true, you could use that to shorten it even more (however I advice to use "true" if you are writing this into a script if only for the sake of readability).
$ while :; do echo foo; sleep 5; done
You could also use any other command that you are sure will end without error (true). In this case you could use this, for example:
$ while sleep 5; do echo foo; done
Whose effect will be slightly different, I bet you can guess what the difference is