Stamina hearts are a measure of energy in cycling, taking the form of three hearts.
If the biker gets tired by pedaling too much, he/she will lose one heart. When the players has no hearts, they will stop for a moment (and consequently lose some places in the race) to drink some water and restore the hearts. While they are renewing their energy, the player will turn blue, as seen in the first picture.
Drafting can help the player to quickly restore lost hearts and to get tired less easily.