Stress is a naturally occurring chemical of Creatures. It most commonly occurs when a Creature is inflicted with pain or is hungry for food.
Stress in all games
Stress is bad for Creatures and occurs when a drive is very high for a long period of time. Usually you'll know when a Creature is stressed because they may have a sleepy face or a mad face.
Stress can occur in these forms:
Hunger for Fat Stress: Stress caused by high need for fat.
Hunger for Protein Stress: Stress caused by high need for protein.
Hunger for Carbohydrate Stress: Stress caused by high need for carbohydrates.
Anger Stress: Caused by high anger drive.
Fear Stress: Caused by high fear drive.
Pain Stress: Caused by high pain drive.
Tired Stress: Caused by high tired drive.
Sleep Stress: Caused by high sleepiness drive.
Crowded Stress: Caused by high crowded drive.
In Creatures 1
Most of the creatures that you see in Creatures 1 are stressed, simply because of a bug where norns find it hard to eat. You're guaranteed to see at least one stressed norn in the game.
In Creatures 2
In C2, you're probably always gonna see the norn sleepy, which makes it stressed. The bug makes it so that norns have a hard time sleeping, but there are some mods that can fix that.