Stamina is one of the most important resources you will have in Story of Seasons: Pioneers of Olive Town. It dictates how much work you can get done around the farm every day, and as the farm grows, you will discover the game is really about a delicate balance of how much work you can do.
You can restore your stamina at the end of each day by sleeping, or you can take a bath during the day to bring some back. This is only ever a small amount, however. Food is a better option for restoring stamina, but once again, there is an upper limit on how much stamina you can get from food each day. Eating a salad will even give you an extra heart, but that will only last for one day.
Fortunately, there is a way to permanently increase your stamina in the game. As you play through the early part of Year 1, you will see some cutscenes of a strange lady walking through the woods around your farm.
Eventually, you will discover who this lady is, and she will tell you that you can visit the shrine at the top of the town to get blessings. You can find the shrine near a large tree in the part in the northeast corner of the town. These blessings cost Sprite Points, and you will be able to unlock a blessing that will increase your stamina. This should happen during the summer of the first year on the farm.
Sprite Points can be earned by doing all your usual chores around the farm, like chopping trees, breaking stones, and weeding.