You're right - there has been a lot of research into this! It takes most people around 15-25 minuites to fall into a state of light sleep known as 'stage 1 sleep' (those sleep researchers in the 50s were great at naming their discoveries!). This is when you get trippy hypnagogic imagery, which if you can remain aware enough for is a complete thrill: fleeting feelings of falling or flying, flashes of landscapes and faces.

After a few minuites of this, you dip further into (you guessed it) stage 2 sleep, where your brainwaves are a bit more evened out and slower - and so on through stage 3, then down into the final and most restful stage of sleep, stage 4 - in stage 4 your brainwaves are slow and deep, your body completely relaxed apart from the occasional signal to move from your brain. In the first 90 minutes of sleep you spend about half an hour here.

After this you rise fairly rapidly into REM sleep, where your classic dreams and classic lucidity take place: Your body is paralyzed, parts of your brain are working overtime and - bizarrely - your body temperature begins to gradually fall. This lasts about 5 mins, then the cycle begins again, give or take the occasional brief awakening (which is usually forgotten). As the night goes on you spend less and less time n stage 4 Non-REM sleep and more in REM.

A good article is here - http://www.lboro.ac.uk/departments/hu/grou...leep/karger.htm - Jim Horne is one of the foremost sleep researchers (if a little controversial). Hope that's enough to satisfy your curiosity!