You might be familiar with sleep apnea, which is a condition in which (one way or another) you stop breathing while asleep, for ten seconds to a few minutes. The cause could be something in your nervous system (we still don't really understand what), or an obstruction in your airways. The result is that you wake up, sometimes with a fight-or-flight (i.e. adrenaline) response. Sleep apnea sufferers wake up several times per night, sometimes not remembering every episode. The net result is that even though, on waking up 'finally', you think you've had a solid 8 hours, you actually had a large number of disrupted sleeps. It's annoying and harmful, and the worst thing is that you might not even know you're a sleep apnea sufferer—when you wake up due to an apnea event, you're typically unaware that it was because you stopped breathing.
This is relevant because, during an apnea episode (and before waking) blood oxygen saturation will drop noticeably. So an easy and accurate way of self-diagnosing is to monitor blood oxygen saturation over the course of the night. Cross-reference those data with a Zeo, say, and you have a cheap, accurate method of self-diagnosis.
I've suspected that I have sleep apnea for a while now. But I don't have an iPhone any more (I sold it and got a Nexus 4), the Tinké doesn't support taking a continuous passive log (since you need to actively press your thumb to it), and I want to do more interesting things with the data than this company's app developers and designers will ever think of, or allow me to. So I guess I'll have to wait until someone else builds something that addresses this need, or build one myself.