Your question was moved to the front of the line because it was more exciting than the others -- plus it had nothing to do with water vapour.

In medical parlance, the twitching is called hypnagogic myoclonus (the first word refers to sleep and the second to muscle twitches).

Myoclonus refers to any kind of muscle twitching. A hiccup, believe it or not, is a kind of myoclonus. It can happen to people at any time, and repeated episodes usually mean there is an underlying medical problem -- multiple sclerosis, ALS, CJD, Alzheimer's or Parkinson's. Anti-depresssants, such as Paxil and Zoloft, can cause myoclonus. Researchers believe a disruption of certain neurotransmitters are related to some forms of myoclonus. One neurotransmitter associated with myoclonus is serotonin, which constricts blood vessels and brings on sleep. The other is gamma-aminobutyic acid (GABA), which helps the brain control the muscles.

Hypnagogic myoclonus (also called sleep starts) is very normal and happens to just about everyone. According to the University of Marburg in Germany, reports of sleep starts are in the 60 to 70 percent range of sleepers (which means everyone) and they're often forgotten. It happens before you're getting into heavy duty sleep mode when the brain is gradually sliding away to dream land. You have probably experienced a falling sensation -- or some other weird feeling -- if you twitch yourself awake. Some people also experience visual sleep starts, a sensation of blinding light. Auditory sleep starts involve a loud snapping noise. (This happens to me, and up until now, I thought it was something outside my body and I would force my husband to get up and go investigate. Guess I won't do that anymore.)

Not a lot is known about why people experience sleep starts, but there seems to be some suggestion that anxiety, some kind of noise, a vivid dream, muscle fatigue, or even genetic disposition plays some part, perhaps in how frequently the sensation occurs.