This question asked about using pulse as a measure of fitness. Resting pulse seems like a nice, simple way to do this. But I find that my resting pulse varies by a huge amount, from values as low as 63 per minute to as high as 83. How much does it normally vary based on time of day, posture (sitting or lying down), or caffeine? If it has this much variation that's hard to eliminate or account for, then it seems like maybe a different measure of fitness would be more appropriate, such as the time required to run two miles. Or if you do use heart rate, what is a better measure of fitness, the average of a series of measurements, or the lowest?
Heart rate is extremely variable, and can go up or down with environmental factors such as heat or cold, foods containing stimulants like caffeine or depressants such as alcohol, standing up versus lying down/sitting, etc.
The two ways that I usually use to recommend for using pulse as a gauge of fitness and/or overtraining, is to take it every morning as soon as you wake up, and after workouts, take it immediately after the workout and again 3-5 minutes later.
The morning one should stay relatively the same, or even drop as your fitness gets better, and the one post workout should drop dramatically within a few minutes. If your morning pulse rate starts going up, or your post exercise rate stays elevated, then you have some factors influencing the rate. This could be dehydration, fatigue, overtraining, life stress, any number of factors.
As an example, my morning pulse rate varies between 45-55, and my working heart rate will usually drop 40-50 beats within 2 minutes (Depending on intensity of workout). As long as you are consistent about when you take it, and track it over time, the trend will tell you if something might be off.
http://eurheartj.oxfordjournals.org/content/27/20/2387.full.pdf (Some discussion of extrinsic vs intrinsic factors)
http://www.colorado.edu/eeb/courses/1230jbasey/abstracts/farah.html - Looking at an exciting picture raised heart rate
@BenCrowell if you're after a few more papers to read have a look at: THE USE OF GPS TO PREDICT ENERGY EXPENDITURE FOR OUTDOOR WALKING, James Michael McKenzie, 2007, p:12-13 specifically: