Well, this data would been incredibly valuable to insurance companies. If you could tie specific demographic information (which Google has mastered) to sleep and exercise routines, this could be used in a risk model.

I'm fairly confident this will eventually happen, as I had a professor explain to me that he wears a fitness tracker in exchange for a premium reduction. I think the incentive of the insurance company is that they found individuals wearing these devices to be more conscientious of their own health (I don't think they request any data at this point).

Even if you don't partake, these risk models will certainly generalize your demographic to their set of data.

From the Ad-revenue perspective, it's easy to see how they could target you for selling melatonin if your sleep pattern supports it. Or, maybe you're active, which brings about plenty of products to target you for.