Seems to be an involuntary physical reaction to a percieved threat, obviously.
“sleep jerks” have also been associated with the “DT’s”. If you guys drink frequently, this may be the cause. Surreal as it is, there is a neurological explaination for it.
The Guardian’s Notes and Queries section gives some worrying possible answers to this:
http://www.guardian.co.uk/notesandqueries/query/0,5753,-21113,00.html
Mind you, they’re not Cecil, just random readers of that particular paper so who knows how accurate they are.
I have nothing to add except a very short yet sad tale.
About 4 nights ago I kicked Ezell the Cat clear across the room after jolting with him at the foot. Poor Boo Boo
I get very similar things to what Phase42 describes… almost like a sort of boom or crackling noise in my head, sometimes accompanied by a flash of light. It’s not exactly a boom that you can hear… what is most noticeable is the sensation of silence afterwards, which makes it feel as if there was a loud boom. Kinda weird, but I’m glad I’m not alone!
This also happens to me sometimes, however, for me it is always in response to some kind of dream stimulus.
I don’t know if any of you will relate to this but:
Sometimes when I am dozing off, and am on the boundary of sleep and wakefulness, I will be dreaming but in some dark far off corner of my mind be kind of aware that I am doing so. Then something will happen, like my foot will slip or someone will try to hit me (or shoot me) or any other kind of stimulus that requires a significant reactionary response… and then I twitch, becoming fully awake, and fully aware that I twitched in response to a dream.
My 0.02.
I thought it was only me. I too experience the same falling or stepping-off-a-larger-than-expected-step sensations that the others have mentioned when taking a light nap (and I quite enjoy the relief experienced after realising I hadn’t fallen).
Perhaps this is why we call it falling asleep? 
Hmm, this is fast becoming a dream-therapy thread.
If by “frequently” you mean having a (single) pina colada last weekend at a BBQ and other than that not being able to remember the last time I had a drink, then that could be the explanation.
Today, that instinct safeguards against involuntary defenestration.
Peace.
Hey, part of my brain is stuck with that word, and if I don’t use it at least ten times in my life, it will have been a waste. (That part of my brain, not my life.)
Anyone here get sleep paralysis? This is also normal and but is more frequent in people with narcolepsy. Its when you are waking up or falling asleep (hynogogic or hypnopompic, depending in the situation), are kind of on the cusp between wakefulness and sleep, but you can’t move. Its VERY scary. I get this frequently. I often will let out a muffled scream (muffled because I am trying to scream, but I can’t actually open my mouth to let out the scream) that my wife hears, then she gets scared and wakes me up. I hate it. Its has to do with the paralysis of your body, controlled by the brain, that occurs during sleep kicking in before you are fully awake or asleep. Sometimes I experience it also as a dream around the time of waking up where I am trying to run from someone, but I just can’t run fast enough to get away. Before they catch me, I wake up.