Signs of Stress
Hamsters can get very stressed when they are put in small cages. You can watch for signs of stress in your hamster. They include:
Bar Biting - This is when your hamster bites on the bars of its cage repeatedly. To stop this, get them a larger cage.
Climbing their Cage - This is when your hamster climbs the walls/bars of its cage. This is a sign that it's bored. Try putting more toys in their cage.
Lethargic - If your hamster is lethargic, then they are probably depressed. Give them more toys or a bigger cage.
Cage Aggression - If your hamster starts becoming very aggressive, this is not a good sign. Keep in mind, however, that the hamster could have babies. This is bad as well though, as breeding hamsters without knowledge of how to do it is never a good thing. To stop Cage Aggression, try to get them a bigger cage