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Having done such a great job of tackling heart health and traditional exercise tracking, now it looks like Apple is looking to new horizons for the next big Apple Watch feature: Sleep tracking.

According to Bloomberg, Apple is already testing a sleep monitor planned for inclusion in “a future version” of the Apple Watch, although sources suggest that it likely won’t be ready until at least 2020, likely for the sixth-generation of the wearable device. The testing has reportedly been ongoing for several months already among testers at “secret sites” near Apple’s headquarters.

Sleep tracking features aren’t at all new, even among wearables — Fitbit already incorporates some of these features into its devices — the move would help to position the Apple Watch as a more well-rounded fitness tracking solution.

In early 2017, Apple also acquired Beddit, a Finnish startup that made a more traditional sleep tracking pad for users’ beds. In an unusual move, Apple also continued selling the company’s products under the Beddit brand, although after the existing stock of units sold out, some wondered whether Apple was discontinuing the product entirely. However an updated version of Beddit appears to have resurfaced in an FCC filing late last year, amidst other rumours that Apple is working on even more sophisticated sleep tracking solutions.

As with most Apple acquisitions, however, it’s a safe bet that Beddit wasn’t purchased merely so Apple could continue selling and developing its products, and many of the engineers behind Beddit have probably been folded into Apple’s other health and fitness teams, and have likely become integral to the work that the company is doing on the Apple Watch.

While “Beddit 3.5” and Apple’s other recent filings suggest that the company is taking a broader approach to sleep tracking technology, it seems like a given that its flagship wearable, which has become the most popular smartwatch in the world, will form a key part of the company’s strategy.

In addition to the sleep tracking technology itself, however, there will be other hurdles Apple will need to overcome in order to make the Apple Watch practical for this kind of use, not the least of which is battery technology. Although some users do in fact wear their Apple Watch to bed, the device was designed to be charged overnight while users sleep — Apple even includes a “nightstand mode” to be displayed when the device is on the charger. For this reason, Apple also has not yet put any emphasis on fast charging for the wearable device; it can take up to 2.5 hours for a discharged Apple Watch to reach a full charge, which isn’t a problem when it’s sitting on your nightstand while you’re sleeping anyway, but would likely be too long for most people’s morning routines.

Competing fitness wearables such as Fitbit advertise the ability to last up to a week on a single charge, although they obviously don’t include the kind of power-hungry features that the Apple Watch does. It’s unclear at this point how Apple will address this, but solutions could include simply increasing the battery capacity, having the sleep tracking run in a specialized “low power mode” to save battery life, or simply offering fast charging so users can pop their Apple Watch onto the charger for shorter periods of time before bed or when they’re getting ready in the morning.