Don't know but I got a nickname from doing that... Space!
Seriously though if this is bothering you there could be something going on. I am not a doctor nor qualified to make any diagnosis. I found these two headings under sleep disorders in the Merck Manual of Medical Information, Home Edition 1997
The Merck Manual defines Narcolepsy:
"An uncommon sleep disorder, narcolepsy is marked by recurring, irrepressible attacks of sleep during normal waking hours, as well as by cataplexy, sleep paralysis, and hallucinations.
The cause of narcolepsy isn't known, but the disorder tends to run in families, suggesting a genetic predisposition. While narcolepsy has no serious medical consequences, it can be frightening and may increase the risk of accidents."
It goes on to say that symptoms begin in healthy adolescents or young adults and:
"The attacks are most likely to occur in monotonous situations such as boring meetings or long periods of highway driving. The person may feel regfreshed on awakening but may fall asleep again several minutes later."
The diagnosis has to be made in a sleep lab. (That reminds me, I am overdue for a sleep lab checkup myself.)
The second heading,
"Parasomnias are vivid dreams and physical activities that occur during sleep."
But these are all during regular sleep. Several subheadings here including sleepwalking, night terrors and nightmares. It says everyone has some of this some of the time.