If there are any sleep tracking functions on the Watch S5, they likely wouldn't be FDA-cleared either. Findings from clinical studies are published before clearance like that happens

Under the hood we can expect the incremental changes that Apple has been giving the Apple Watches in the past. The Series 3 has Apple's dual-core S3 chip and W2 wireless chip, while the Series 4 has an S4 chip with a dual-core 64 bit processor

Under the hood we can expect the incremental changes that Apple has been giving the Apple Watches in the past. The Series 3 has Apple's dual-core S3 chip and W2 wireless chip, while the Series 4 has an S4 chip with a dual-core 64 bit processor

Both the Series 2 and 3 are water resistant to 50 metres, both also have a faster dual-core processor and GPU, and a brighter display compared to the original Apple Watch. 

Announced alongside the iPhone XS Max at Apple's September event, the Apple Watch Series 4 is the first time the design and size of the Watch has changed since it was first released.

Along with that design change comes a bigger size of screen that reaches closer to the edges, a slightly thinner body, and a potentially huge new heart sensor.

This is really useful for people who use their Apple Watch for running, and hate taking their iPhones with them – you'll even be able to stream music from Apple Music.

If you're looking for an Apple smartwatch, and aren't fussed about having the latest model, the Apple Watch Series 3 should be the one to go for.

Exactly the same housing as the original model but boosted with a dual-core processor and comes with watchOS 3 out of the box. Apple clearly thought the original needed that extra bit of power for tasks so it’s a welcome change, but you don’t get as many options as you did with the very first Watch.