Since WatchOS 9 and the ability to track sleep phases I’ve seen this seems to be inaccurate, it reports me being in core sleep when I was clearly awake albeit lay down, also it tells me I’ve been awake when I’ve been asleep. I had a £99 FitBit that seemed more accurate than the Apple Watch.
Apple Watch Series 7
Hello AndyC44,
You may find some helpful info and steps for tracking your sleep here: Track your sleep on Apple Watch and use Sleep on iPhone - Apple Support
Get more accurate results with your Apple Watch
If you paired your Apple Watch after completing Sleep setup, you can still turn on Track Sleep with Apple Watch. In the Watch app on your iPhone, tap the My Watch tab, then tap Sleep. Then tap Track Sleep with Apple Watch to turn on this setting. To get the most accurate results when wearing your watch to sleep:
• Turn on Charging Reminders. On your iPhone, open the Watch app and tap the My Watch tab, then tap Sleep. Tap Charging Reminders to turn on a reminder on your Apple Watch to charge before your Wind Down time. If your watch battery dies, it won't track your sleep data.
• Make sure that you're wearing your Apple Watch at a comfortable fit. If your watch is too loose, the accelerometer might register too much movement during your natural sleep.
If you continue to have issues we'd advise reaching out to the Apple Support Advisors that specialize in the feature here: Apple Support
Cheers.
Inaccurate Sleep Data
