The author also links to his student's analysis of a bunch of sleep trackers: <a href="http://sleep.cs.brown.edu/comparison/" rel="nofollow">http://sleep.cs.brown.edu/comparison/</a> which is quite interesting; the TL;DR is: it pretty much doesn't matter which one you choose; the data aren't particularly scientific but can be fun to look at.
Been waiting for an API for years now, sadly don't expect one to come anytime soon. Luckily their private API is standards based and has year+ access tokens!
Put this into a quick ruby class if anyone is interested: <a href="https://github.com/fredkelly/sense-client" rel="nofollow">https://github.com/fredkelly/sense-client</a>
If you're looking for a cheap, hackable device with basic sleep tracking, I highly recommend the Xiaomi Mi Band (1 or 2). The protocol has been reverse engineered, so you can use a program like GadgetBridge to get all of your raw data out of the device.