Lark contains a micromotion sleep pattern sensor that records thousands of data points every minute about your movements during sleep. Using actigraphy algorithms developed by sleep researchers, Lark then determines your sleep quality and when you're sleeping, falling asleep, awake and tossing and turning. Actigraphy is the most accurate way to monitor sleep patterns without actually measuring brainwaves, and is about 85-95% accurate for healthy adults. Actigraphy is not, however, well suited to monitoring sleep stages such as deep sleep or REM sleep which is scientifically determinable only with brainwaves. Contrary to popular belief, adding additional sensors to the Lark wristband will not improve its accuracy.
For more information on actigraphy, please see this recent article.