Stamina is a game concept in Harvest Moon 64.
Stamina is how much energy Pete has to do actions.
Stamina is used when any tool is used. Upgraded tools use more stamina than regular tools. When your stamina reaches 0, you will not be able to use any tools.
Eating and drinking different items restores stamina a small amount. Making an offering to the Harvest Goddess and selecting 'Strength' restores your stamina to the maximum. Manually going to sleep also restores stamina to the maximum, but not going to sleep only restores it +20.
When you start the game, your stamina is at 100. To increase this, you have to collect Power Berries. Each Power Berry adds +15 to your maximum stamina.