Use a Fitbit or another watch such as an Apple Watch to track your heart beat. The secret way to know when you need to take it is easy is now quantifiable. Use your watch.
Establish your normal resting heart rate. When you wake up, if your resting heart rate is higher than normal, then cut off the day’s intervals or take a rest day. For instance, if I wake up and my heart rate is 65, then I will reduce the work load. My average heart rate is 60. My body is recovering and needs time to make adaptations.