This is normal in most cases, it is similar to postural hypertension in which the sudden act of standing can increase one's blood pressure and heart rate. If you are otherwise healthy, a heart rate in the upper 50s is a sign of a very efficient heart.
Your heart is slower than the average, probably due to fitness. You will have a larger stroke volume if this is the case, which is why it can beat slower. Basically it pumps a higher volume of blood with each beat than the average person. When you stand, you lose a lot of your blood into you legs, due to gravity, and you have to wait for the next beat to start getting blood back up to the brain. So in real basic terms, you stand up, feel a drop in blood pressure (postural hypotension) and then your heart builds the pressure back up which can take 2 seconds with a low pulse rate. This is common with athletes and getting up more slowly helps in most cases. If your lower heart rate is not due to fitness, then I would seek medical help. If for example you have recently started any medications such as beta blockers, this can be the cause.
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