Why do we wake up when our dreams get too scary?

It always seems to happen doesn’t it… we’ll be happily plodding along through a dream and then we hit a scary moment…it gets worse…we begin to feel it…really feel scared… perhaps it gets even worse…and then BANG you wake up!

I’m wondering why? and whether or not this has any implications on various philosophical theory of emotions….

and then whether I can be clever and weave it into my essay at all to prove my point…but you guys need not worry about that part :-)...

But anyway…any ideas why we wake up at this point? Or does anyone continue to sleep through nightmares…they finish..you carry on sleeping…and you wake up in the morning later…thinking “by God that was a terrifying dream I had last night” but not wake up directly in it….?