I had been eyeing up the SleepTracker watch, and was pleasantly surprised to receive it as a present for my birthday from my girlfriend a few months back. The concept is genius: Why wake up in the middle of deep sleep, when you can be awakened intelligently at the right time?
Sleep happens in multiple stages: Drowsiness, light sleep, deep sleep, and dream (or REM) sleep. This cycle repeats multiple times in a night, and usually lasts 70 to 90 minutes. You can read more about sleep stages at helpguide.org.
Ever feel like you woke up at "just the right time"? After each cycle, you are returned to an "almost awake" state. When you wake up at this point in time, you feel much less drowsy, and much more refreshed than when you wake up during deep sleep.
That's the point of the watch: to monitor when you move (when you are almost awake), and wake you up at the right time in a given window. Ideally, you feel refreshed every morning, ready to jump out of bed.
How well does it work? Surprisingly well. After several weeks of use, I was almost always woken up during my set window. Waking up in an "almost awake" state is much more enjoyable than being rudely woken up by an alarm, and you do feel like you're woken up at the optimal moment.
What does it not do? It won't magically make you less tired. Just waking up at the right moment doesn't turn 5 hours of sleep into 8. You can shake off that "sleepy" feeling much, much faster, but you'll still need just as much sleep as before. It may be tempting to set the window to the maximum 90 minutes so that you're always woken up at an "almost awake" moment, but I found that sometimes the extra 90 minutes of sleep was worth it, even if I wasn't woken up at the perfect time.
As an added bonus, the SleepTracker Pro connects via USB to a computer to let you download your sleep data in text, csv or xml format, which can be almost (or more!) interesting than its intended purpose. With a bit of digging around on their forums, you can even find a perl script that works (easily) under Linux and (with a bit of effort) under OS X.
I've uploaded a couple weeks worth of my sleep data:
Here's the perl script (sleeptracker.pl) as well as a helper script I wrote (trackmysleep.pl):