I use an Apple Watch and an app called Pillow and it does a plausible job of guessing when I went to sleep and when I woke up. It also identifies light/deep sleep and REM, but I have no way of telling how accurate. It seems to detect awake periods at night as well as I can remember them.

But ... I kinda assume it's a crapshoot and I use it more as a novelty and an estimate of total sleep than any kind of accurate representation of sleep cycles.