I made a table to compare Basis Peak replacement options.
I'm most concerned with good sleep tracking (including phases) and 24x7 HR with data exportable. (I used QuantifiedBob's scripts to get the Basis data.) But I also appreciate the simple smartwatch features of alerts and music controls.
Several of the alternatives seem to have the promise of maybe being able to get at the data through APIs of their respective app ecosystems. Those that are Android Wear (eg, Polar M600) in particular seem to have things like Heart Rate OS already available (and still under continuing development/improvement). I haven't dived down into those details yet, but this quote is promising, "Heart Rate OS can be configured to monitor your HR 24/7 at intervals with averages of your choice and sync that to google fit." Android Wear seems like it'd probably be easier to develop for and/or at least be more likely to be long-lived than Garmin or Pebble's development ecosystems. Not sure about the M600's sleep tracking out-of-the-box, but I'm hopeful Android Wear 3rd-party apps might help if needed.
The Philips Health Watch got some love from one reviewer on one of the Basis website's own replacement rec threads but seemed lacking to me vs. the other options for various reasons, like no smart-watch features, but more importantly because of no functionality-extending app ecosystem, and most importantly there seem to be many reports saying the automatic sleep tracking wasn't working (on the top Google review and on Amazon). Also not so waterproof.
It's nice that the Jawbone UP3 has the extra sensors the way the Basis Peak does (which I presume both used for more accurate sleep analysis), and many reports suggest it is better at sleep than most of the others, so I'm testing that now.
There are also a host of sleep-only alternatives that could be used in addition to a worn activity tracker. There also seem to be many papers scientifically validating the under-matttress approach to sleep analysis (Beddit, Aura, etc.), even comparing favorably to standard EEG. The Emfit QS in particular seems to have the highest quality sensor in the category of devices you install in your room and then they track sleep automatically without needing you to do anything, good enough to even do HRV during sleep. It also has the best Amazon reviews of any of the sleep trackers I've found. I might try that out.
There is also a small spearhead of headband consumer EEG devices (Neuroon, Sleep Shepherd Blue), notable as being the first true potential heirs to the Zeo. Both are worn on the head and both actually do EEG and so theoretically should be able to tell sleep state the same way as a the gold standard sleep study. Both appear from reviews to have some usability issues that makes them far from universally loved, so maybe they aren't ready for prime time, but OTOH a lot of people seem to really like them. They both also do a few other things to help make sleep better rather than just passively track.
It's also worth knowing that there are a bunch of not-consumer-targeted but rather doctor-ordered (only, I think) devices that are supposed to be sort of home-based alternatives to going into a sleep lab. They are not cheap and don't seem easy to buy or compare, but here's an interesting link on them.
I never did flesh out every cell in my table, but I'm hoping some combination of UP3, M600, and/or Emfit QS might end up being better than the Peak.