Further to my previous comment, I'd also like to point Apple Watch users to an app called Cardiogram, which is (I think) better than Heartwatch. Both are good for different reasons. You can set Cardiogram to record continually. I havent done that, I'm sure it would eat battery.
You can see from the graph that its checking is frequent. The small blue bars are indicators of my pulse dropping under 60. Sometimes it goes under 50. But, I need it to just keep an eye on me. Prior to being treated for my heart issues, I was getting all kinds of crazy readings. That is, heartrate way too fast for good health. For people like me, Apple watch combined with appropriate apps is a godsend.