I got the Fitbit One. You have to manually tell it you're about to sleep and you just woke up, then it tells you how long you stayed in bed, how many times you woke up, and how much deep sleep you had.

Based on that, I averaged that I needed 8 hours of in-bed time to get 7 hours of deep sleep, and then I had no need for the device anymore. Until my sleep patterns change.