Rather than sensing sympathetic nerve activity in the skin, it's possible that Sleeptracker senses body movement instead (assuming that increased body movement is related to lighter stages of sleep).
Disclaimer: Sleep ain't my specialty. This is speculation.
Other ways of wakening: the Digital SunRise Clock by Bio-Brite wakes you with a light that slowly brightens over 20 - 30 minutes. I've used it for the last year. My review: thumbs up.