... And suddenly, it's over again. Your heart is pounding. You look around the room blinking, shocked to find that your brains aren't splattered all over the bedroom. Physically, you appear perfectly fine.
Congratulations, you've just experienced a random occurrence of Exploding Head Syndrome and, yes, it's totally a real thing.
Wait, What's Going On?
Well, no one is really sure. Physicians think there's a link to stress or extreme fatigue (surely you've heard the common saying, "Man, I'm so tired, my brain is going to fucking explode!"). They think physically it may also be caused by an inner ear problem, or possibly a form of minor seizure in the temporal lobe. But don't worry, according to Wikipedia, "it is not thought to be dangerous ."
The noise itself can be an explosion, or a roar, or waves, or a gunshot, or an electrical zapping. But it will always be loud, like pants-shittingly loud, and it's not a dream. We're just barely exaggerating about the pants-shitting thing, too--the experience is said to be followed by a rush of adrenaline, an elevated heart rate and terror (well duh).
It may only happen once, it may happen several times. EHS is usually random, even happening when a person is fully awake. Whoa, what if it happened right when you were in the middle of defusing a bomb?