Sounds creepy, doesn't it? It's true though, for virtually all endurance athletes: runners, cyclists, xc skiiers, swimmers et al.. The reasoning is pretty straight-forward. Endurance athletes strengthen their hearts and they expand their lungs and circulatory vessels by way of prolonged and regular exertion (exercise or training). In the process, they also "teach" their bodies to be efficient, to do "more with less" (oxygen). This occurs at the cellular level. Hence, as the body's efficiency improves, the muscles require less and less oxygen to produce the same energy output. Oxygen is carried to the muscles in blood vessels which have grown a little bit in size, making the blood flow more efficiently. When the volume of oxygen enriched blood required by the efficient muscles decreases (a positive effect of exercise) the heart can do the same job with fewer beats. Since the heart and blood vessels are also enhanced in a trained athlete, the diminished oxygen requirement compounds the heart's "slow-down". For these reasons, athletes such as Lance Armstrong have resting heart rates in the 40's even in the high 30s. ("Resting heart rate" usually means the groggy moments right after waking up in the morning)
Last night I wore my Polar heart rate monitor to bed and recorded my heartbeat for the night. This morning when I checked the results, the average heart rate during my sleep was 58, and for much of the time, my heart rate was in the 48-52 range. It spiked about 6-8 times to the 70s for whatever reason... One intersting thing I noticed is that in the hour or so after I hit the 'snooze' button on the alarm clock, my heart rate dropped noticably lower. Why am I not surprised?
So hopefully I've explained why a lower heart rate is a desirable result of fitness training for any athlete. In addition to being an overall indication of fitness, it's also a good way to judge whether an athlete is rested or over-trained. An elevated resting heart rate means the body is tired and/or in repair from the 'controlled injury' which is inflicted in training.