I've been interested in tracking my own sleep too. I wonder how useful these commercial products are in producing data that is correct. They come from technology companies who primarily want to collect data, as much data as possible. They're not at all interested in providing their users with an honest interpretation of anything. A lot of them probably don't even have the expertise to interpret whatever they've gathered.

I've been involved in sleep studies in the past and none of them involved using a smart-watch or other commercial wearable. They used electrodes (?) that were stuck to my body and fed into a computer. What data was being collected and how to properly interpret that data is beyond me.

Making a DIY device is probably feasible, but again, I wouldn't have a clue on how to interpret the data gathered. I don't think it'd be too complicated though.

Tools built specifically to monitor one signal are probably better than a generalized thing, such as a smartwatch, from a company that produces surveillance equipment for marketing purposes.