Hello!
In the french LD4all, danboss find a apps for the iphone that show the sleep cycle. I buy it, and it work perfectly for me. You put your iphone close of your pilow and : Good Night. With your move in the night, the app can know in witch sleep cycle you are!
The app can also wake you in the good cycle so without tiredness.
This is the night of danboss : https://www.imagup.com/pics/1261710269.html
Alaky 