Lowered Starting Health is one of the primary features of Tough As Nails. Depending on the difficulty level, you will start with less health than normal.
Peaceful: 10
Easy: 7
Normal: 5
Hard: 3
You can increase your hearts by finding Lifeblood Crystals in Nether Fortresses. You can still only have a maximum of 10 hearts, but the lowered starting health adds an additional layer of difficulty to the game.