As you use your tools you will use up Stamina. Depending on how much stamina you have also depends on how long you will be able to work. If you turn blue from over work you will either have to eat some food, sit in the hotsprings, go to bed or go to the clinic.
Your Stamina is split into two parts: your Strength and your Fatigue. The Strength starts out at a high value and goes down to zero, while your Fatigue starts at zero and goes upwards. The Stamina is also based on points. At the start of your game you will have 150 Strength and 100 Fatigue. You can increase your Strength by finding the Power Berries that are hidden in the game. Each berry will add 10 points to your total Strength. If you find all 10 berries then you'll have 250 points of Strength. You don't have any way of increasing the number of Fatigue points you have. The only berry that will help your Fatigue is the special Blue Berry you can receive from Kappa by throwing 10 cucmbers to him in the lake over 10 days. The Blue power Berry will allow you to work in the rain without the weather affecting your Fatigue points.
When you use a tool you'll decrease your Strength points by a certain amount. Most of the tools only affect your Strength a little bit, but tools like the Cursed ones will use up a large amount of Strength points every time you use it. Your farmer will give you a general indication of how many Strength points he has when you are at 50%, 20%, 5%, and 0 strength points.
After you finish using up your Strength points, you can still work because your Fatigue points start to add up. Normally your Fatigue points won't start adding up until you use up your Strength, but there's a few things that begin increasing your Fatigue before using up your Strength. If you work outside when it's raining (or snowing) and you don't have the Blue Berry from Kappa, your Fatigue points will increase at the same time your Strength points decrease. Your Fatigue will also increase if you use one of your tools after 10pm, haven't gone to bed and it's after midnight, or eat a Poison Mushroom or the Red Grass you find on Mother's Hill. When your Fatigue hits 100 you'll turn blue, even if you have a lot of Strength points left.
To help keep your Stamina, you can use the items you have inside your house.Depending on how many hours you sleep, you'll recharge you Strength and Fatigue a good amount. Putting a Moon Drop or Blue Magic Grass flower in the Flower Vase which you can buy from Won for 2000G will help regain more Strength when you sleep. If you put a Pink Cat, Red Magic Grass, or Toy Flower in the vase, it will affect your Fatigue. Since you can't put any flowers in the vase during Winter you can put a piece of wood in the Fireplace to help keep your Fatigue. To get the fireplace get Gotz to upgrde your house to the highest level. Do not put a piece of wood in the Fireplace during any other season because it has the opposite affect on your Fatigue!
To recharge your Stamina right away, you can eat food. Different food items recharge different amounts. The best food to eat is the Elli leaves [See the recipies section] with you when you go into the mines. You can also buy food at the shops in Mineral Town, or cook some yourself. The more ingredients you add to a basic recipe, the more it will affect your Stamina when you eat it. If you have made the basic recipe before and are adding more ingredients, you'll be told how much SR (Strength Recovery) and FR (Fatigue Recovery) the new recipie has compared to what you had made before hand.