This app was mentioned in 2 comments, with an average of 1.00 upvote
@harvinattack33: it was a permission problem, I removed one deprecated one, now it seems to work on everyone's phone, and I've released it publicly https://play.google.com/store/apps/details?id=com.sleepalert.heartrate
Hope my app be of service to you, SleepWake, alert when you doze off in lectures ;), and keep you safe on road trips https://play.google.com/store/apps/details?id=com.sleepalert.heartrate