Now, I had no doubt that the SLEEPTRACKER would perform just fine as a digital watch, which it did. Setup was simple, and after it was done, I could tell the time and date. However, this watch sells because it wakes you up like you have never been woken up before. I went to bed that evening at the time that I told the SLEEPTRACKER that I would be in bed by. During the night, my baby woke up crying, which in turn woke me up. I remember getting up at about 2:10 AM to calm him down and get him back to sleep. In the morning, I heard the alarm go off. I checked the watch, and it was 5:47 AM. Oh, and yes – I felt perfectly awake and satisfied with the amount of sleep I got. I didn’t feel the need to hit a snooze button of any sort.
After I got up, and did the toothbrush thing, I checked the sleep data. It was very interesting to see the times that the SLEEPTRACKER had recognized as my light sleep/awake moments. Most intriguing though, was that it marked 2:11 AM as one of those moments – right after my son woke up crying!
When I was in college, friend and fellow pre-mudphud Matt D used to recommend sleeping in 90 minute increments, because that was the approximate timing between REM cycles. The trouble was always individual variation -- if your cycles are 95 minutes, or fluctuate throught the night, setting the alarm properly becomes impossible.
This watch seems to do the trick. It's not clear how the Sleeptracker works -- whether the underside sensors are monitoring the sleeper's pulse, or arm movements, or temperature, or some combination. But, as the reviewer mentioned, you can review your night's sleep and tally the "almost awake" periods. Maybe you can reverse-engineer their sensors with a Holter monitor or insomniac partner.
(A good primer to the physiology behind waking is Akerstedt and Shulz's review article.)