Your Resting Heart Rate is your heart rate while awake where your body needs the least amount of blood to function — a great measurement of your heart's efficiency. Your resting heart rate is crucially different than your sleep heart rate, however.
In watchOS 4, Apple started tracking your resting heart rate, however, Apple does not intend its users to sleep with their Apple Watch as of 2017. Therefore, the resting heart rate in Health becomes skewed towards your sleep heart rate, which is lower than what your resting heart rate is suppose to be (during sleep, even less blood is needed to function as the body is in a sleeping state).
Because of this, Sleep Pulse accurately measures your resting heart rate while your sleep is processing after you tap "Wake" each morning — ensuring that your resting heart rate measurement is taken in an awake but resting state before the start of your day.