Why is my character is exhausted?

Whenever you gather resources in the wilderness or in your own backyard, your Vigor drops by a bit. There are three stages of your character’s vigor level that may affect your mastery gain. Your character has 600 vigor though I’m not sure yet if this can be increased permanently in any way. You can check your character’s vigor and other effects by clicking any bar/gauge on the top left corner of the screen.

It's also important to know that if your Gathering Mastery level is capped, you can still gather resources and your vigor will not decrease.

Gathering costs x2 Vigor and gives x2 Gathering Mastery level

Gathering costs x1 Vigor and gives x1 Gathering Mastery level

When exhausted, your Gathering Mastery growth rate will be reduced to 10% (this is quite severe). You can still gather the same amount of resources even while in this state.

How to recover your Vigor

The other method is to wait for your vigor to be replenished automatically, at a rate of 10 vigor every 10 minutes. This means to get a full recovery, you have to wait for 10 hours. This can be considered a limiting factor on how long you can play the game straight.

As long as your vigor is reduced, the game will start automatically replenishing it. One good tactic to maximize your vigor recovery is to log in the game regularly, harvest wood or stone in safe areas like the Fall Forest, log off and repeat after a few hours.