It is depicted on the character's Inventory screen as a blue-red gradient bar split into 7 sections. Each section represents one in-game day, i.e. a character can survive for 7 in-game days without sleep before they die.
A character's Fatigue begins to be affected after two days without sleep and becomes more apparent the longer they do not rest.
Reducing Sleep Deprivation
When a character enters a building from their own community, they will automatically sleep if they are tired enough (at least 1/2 day sleep deprived). They will otherwise say that they are tired and head to the nearest player community building to go to sleep. If they are at least 2 days sleep-deprived, they may also sleep in random houses that are not part of the character's community.