It’s very useful to have some way of monitoring your heart rate during a ride. I generally always wear a heart rate transmitter and record my rides. Over time, this gives me a good insight into the state of my health. For instance, the day before I came down with a bad cold just before Christmas, I noticed that my heart rate was about 5 beats a minute higher than normal on my daily commute. I deduced that I was coming down with something, and was able to react by increasing my vitamin C. I did get that cold, I did miss one day of work, but I was back riding and working 48 hours later.
But the monitoring delivers more than a heads-up on the state of the body, it can also provide good insight into personal energy. My commutes take between 250 and 500 calories, depending on wind, and temperature. If I see that the wind will be blowing stiffly into my face on my generally uphill homeward commute, I know I should have a few more calories in the tank before I head home, so I will manage to get a snack into me with roughly the required number of calories early enough that the ride is manageable.
Living a cycling lifestyle has kept my weight low and stable, and focused my attention on the quality and energy content of what I eat. Natural snacks, like raisins, become equated to kilometers cycled.