I use the Polar H10 HRV monitor with Polar Beat while training and Elite HRV for my morning relativity reading.
I have been tracking my HRV for over 6 months now.
Each morning you take a base reading on yourself. Before fully waking up, you want to get this reading as a blueprint for how to approach training that day. If your heart is in a state of recovery from the stress of the day before you'll want to take it easy. Pushing yourself while your body is already compromised is a sure way to crash and burn. Over-training is very real, and it can easily be fixed with a few simple methods.
First, you should avoid working out so hard that you can't train the next day. The goal is consistency, not maxing out whatever arbitrary record you've set against yourself. Our muscle fibers can still be torn without completely shocking our nervous system. It's not only taxing on the body, but willing yourself through a workout you were sore at the start of is nothing short of torture. It's hard enough to wake up and get through a work day, let alone pushing an hours worth of sweat out of every pore afterward. Find your pace.
Bringing me to my second point, nutrition. Less processed sugar, more wholesome and clean food. I don't need to tell you eggs and turkey bacon is healthier than organic doughnuts and Starbucks lattes. The fuel you use will effect any machine you're trying to operate, whether human or otherwise. Put Mountain Dew in a lawnmower and see how much grass you cut. I don't think anyone really likes spinach, kale, and wheat grass shots, it's just what happens to be good for us...
I am a huge proponent of fasting, it gives me the health results I want, yet I can still indulge in the sweeter things. After all the other healthier macros have been preloaded into my stomach I can finally crack open a beer and eat some ice cream. When the stomach is empty it shrinks and opens the Pyloric valve. This muscle is directly responsible for instantly compressing and passing whatever you ate as a first step in digestion. If you are sending in bad food, the Pyloric valve releases bad food enzymes. If you send in good food, it's first response is good food enzymes. Resulting in the formation of our eating habits. Simply put, if you eat well you will develop a taste for the healthier options. Over time. This is not a quick fix.
This all brings me back to taking HRV metrics. By taking a morning reading you can set yourself up for training that day. If your number is low 1-5, take it easy. Go for a walk a walk and stretch.. At a 6-10 you can take on the day with much more vigor. You will inherently see the way certain foods and drinking alcohol effects this number. Living healthy is simple once we form our day around it. Take care of your heart, it's always taking care of you. <3