Normally, the heart rate slows during sleep at night; this is called the nocturnal 'dip'. Those with very small or no dips in sleeping heart rate were about 2½ times as likely to die in the next 7 years as those with normal or large dips during sleep, as detailed in this study.
In the study above, average awake and average sleep heart rates were used to determine "non-dipper" patients, defined as follows:
(awake value - sleep value)/awake value < 0.1
In other words, those with a sleep dip of less than 10% were more likely to die in the next 7 years. In normal subjects, the heart rate dips during sleep by at least 10%. And if the "dip" is significantly higher than 10%, you can count yourself as a very fit individual!
Now, where is that heart rate monitor of mine!