Why did people get tired when they have nightmares?

Yeh...like the title:
Why did people wake up tired (like headaches, fast heartbeat rates, sweating) when they had nightmares??
Kindly enlighten.

I'm no sleeping expert, but my personal guess is this:

When one has a nightmare, they do not get the normal night's rest they would usually have. Instead they are fixated on a terrifying/horrible situation, which causes stress. Stress can make one tired and weary. The fast heartbeat, sweating, etc is most likely due to the fact that what they are experiencing is seemingly real, thereby creating feelings which would, in turn, cause the individual to become very ''on edge''. We've all experienced moments in our lives that cause fear in some form; our heartbeats go crazy, and when the body is trying that hard to keep ''stable'', blood is being pumped around the body at a high rate - which I would imagine would produce heat, as well. This could explain the sweating.

Then how come a very active dream (like being chased or running all the time) isn't considered tiring?
I thought the situation is almost the same as from a nightmare..

Perhaps because nightmares prematurely end your sleep, whereas normal dreams do not? As stated, I'm no expert, so don't take my word for doctrine. :D

ok thanks anyway...all opinions accepted. ;)