If I wake up on my own in the middle of the night, I am awake enough to read until I feel sleepy again.
If something wakes me up (loud noise, phone ringing, kids calling out etc), I wake up enough to deal with whatever it is whilst still feeling a bit dozy, fall asleep again almost immediately after, then wake up fully about 5 mins later to not be able to get to sleep easily. I presume it is adrenaline for the first bit, not sure what it is for the second part. It's all a bit perculiar (sp).