The YouTube channel, 'The Quantified Scientist', does a great job of comparing sleep tracking wearables to a reference EEG. He collects a lot of data and presents it very well.
Thank you for posting this! I recently started using my Apple Watch for sleep tracking and have been dismayed at my quality and quantity (vs. "bed time") of sleep. Searching the channel for "sleep" I instantly found 3 videos for my "to watch" list.