why not just get a guardian that DOESN'T fall asleep?

[Steve Vander Ark]

No: HPFGUIDX 19159

So each defense of the stone is non-lethal, each one can be defeated 
by the right combination of brains and brawn and skills. These traps 
were all placed by the good guys, right? Okay, here's my shot at a 
reason. 

If you create lethal traps, that puts you in the category of Dark 
Wizards. If you just put fire there when you know it's going to kill 
whomever tries to get through, your intention is to kill that person. 
Intention is key to all the magic in the books, so if the intention 
you put into your spell/trap/puzzle is absolutely lethal, you are 
using Dark Magic. Now I'm sure that someone like Snape wouldn't have 
a lot of qualms about using Dark Magic for a good cause, but I'll bet 
Dumbledore wouldn't hear of it. So they had to create traps which 
weren't intended to kill, oh no, they're intended to "thwart 
progress." That's a very different thing, without any hint of Dark 
intent. 

Flitwick gets right into the spirit of the thing and creates 
something bright and colorful and at the same time damn near 
impossible to figure out, so upon reflection he includes the brooms, 
since then the intention not to harm is clear, even though he can be 
pretty sure no one will actually be able to catch the correct key. As 
for Devils Snare, Fluffy, and the Troll, well they're not evil, 
they're simply animals who attack, which isnt' the same thing. And 
none of them, I would argue, are technically Dark Creatures, so 
they're the perfect choice: nasty but not Dark. So far we're doing 
just fine within Dumbledore's Round Table-ish code of honor.

Well, then it's Snape's turn. He also follows the rules and makes the 
riddle breakable, but just barely, and he includes a few nasty 
poisons in the mix just because he just wants to so badly. Dumbledore 
shakes his head and suggests that a little butterbeer would be a bit 
less Dark, but Snape insists. 

So there you have it. No one should be able to get past these traps, 
but then several people do, including a gang of kids, and when Flamel 
hears about this, knowing these traps were about as good as any could 
be and not be Dark, he finaly agrees to give up the stone and have it 
detroyed.

Steve Vander Ark
The Harry Potter Lexicon
http://www.i2k.com/~svderark/lexicon