Athletes and other very fit people may feel dizzy when they rise from lying to standing because of their slow pulse rates. Exercise makes your heart stronger so it can pump more blood with each beat and it doesn't have to beat as often. A slow pulse rate can be good. Since your heart doesn't beat as often, it has more time to rest between beats. Like a low-mileage used car, perhaps this will mean it takes longer to wear out. But a slow heart rate can make you dizzy when you change position.
When you raise yourself from lying to sitting, or from sitting to standing, the force of gravity pulls blood down from your brain towards your feet and your blood can't get back to your brain until your next heart beat. If you have a pulse rate of only 50 beats a minute, it will take more than a second between beats. That can be enough time for your brain to suffer briefly from a lack of oxygen, so you feel dizzy. You can even pass out while you wait for your next heartbeat to come along and pump blood back up to your brain.
Dizziness can also be a sign of an irregular heartbeat or blocked arteries leading to your brain, so people who feel dizzy when they get up should check with their doctors. If they are athletes, chances are that they only have a strong athletic heart with a slow rate, and all they need to do is remember to get up slowly.