Apnea Sleep Assistant

Put your smartphone on your chest with a sport belt and sleep with it. It will detect when you are not sleeping on one side and will vibrate first. If you don't move, it will start the alarm. All parameters are configurable.

Some users are using the next waistband, you can buy it on amazon for 10 $:

https://www.amazon.com/Cell-Phone-Waistband-Cellphone-Sweatproof/dp/B01AARJ4SQ/ref=sr_1_4?ie=UTF8&qid=1504016464&sr=8-4&keywords=Sports+phone+belt