A Good Way to Gauge Recovery
Heart rate variability (HRV) is an excellent way to check your response to stress. There's considerable research on this topic and many devices available to do this.
HRV is the measurement of time between each heart beat, and this is highly variable. Since our response to stress activates the sympathetic nervous system, this will result in the release of cortisol and other hormones that increase heart rate and blood pressure.
When this happens, the time between each heart beat becomes more regular, improving your fight-or-flight ability. On the other hand, when we recover from stress, the parasympathetic nervous system is stimulated. This restores the healthy variability between each heartbeat.
This information tells us a lot about our recovery from training and stress in general. When our HRV is high, it's a green light that our nervous system is in balance between stress and recovery. When our HRV is low, this indicates a low tolerance to stress, poor recovery from exercise, and possible inflammation.
HRV can change on a daily basis. When used with other markers of well-being such as sleep, mood, and energy, this can be a great tool to plan workout intensity, recovery days, and nutrition needs.
There are a number of devices out there you can purchase to test HRV. Essentially these help determine your baseline and then they tell you when you're in sympathetic or parasympathetic dominance. These are often color coded. So green means you're good to go, orange means you're needing some recovery – maybe an easier workout day, or some light mobility/stretching. Red means you need to take a rest day until you're back in green again.
When you go into sympathetic dominance, unless it's extremely high, usually backing off on the training or doing short easy workouts for a couple days will afford you enough recovery.
When you're in parasympathetic dominance the best solution is some proper rest for full recovery. Often the scores will slowly get higher and then suddenly plummet into parasympathetic dominance and this can be tricky to get out of. Interestingly, it often precedes the onset of flu or some other illness.