Many of the commonly kept hamster species come from areas which are relatively cool during the hamsters' normal waking hours. The Syrian Hamster, for example, is active at night when temperatures can be sub-zero in their native land.
There are two main effects of heat
The first, which can be frightening to the owner but not a major problem to the hamster, is sometimes called Sleeper Disease. When the external temperature is high, say 75 degrees farenheit, the hamster appears lifeless and floppy. It can be picked up, turned over and shaken, with no apparent sign of life. If observed carefully the whiskers can normally be seen twitching. If the animal is held gently, then normally it will awaken within a few minutes. This awakening can be very abrupt - so hold the hamster over your lap or a table in case it moves very suddenly. This condition occurs commonly in young hamsters kept together when one or more may suddenly appear listless.
This Sleeper Disease should not be confused with the very serious heatstroke described below. Any animals not coming round within 5 minutes should be treated as if they have heatstroke.
Hamsters can very quickly suffer from heatstroke. Danger areas are in cars, where a short period in a stationary vehicle - even five minutes - can kill, and in sheds where many hamsteries are housed. Another problem is the siting of a cage in direct sunlight in a room.
The symptoms are as described above for Sleeper Disease but the hamster does not recover in a few minutes. Hamsters starting to suffer from heatstroke will often dribble and saliva will be apparent around the mouth. They will move sluggishly and will eventually lie quietly on the floor of the cage and then die.
It is vital to cool the animal down as quickly as possible. Splash the animal with cold water to bring the body temperature down and try to give cold water to drink to replace lost body fluids. This will be sufficient if the animal is just starting to suffer from heatstroke. If there is no immediate sign of recovery contact your Veterinary Surgeon for advice and possible treatment. The one time I did have hamsters suffering from heatstroke I was advised to put them in the fridge for a while (NOT the freezer) to get their temperature down. The Vet then gave an injection, one survived, one died, the other 20+ hamsters were unaffected. It is difficult to treat hamsters with heatstroke successfully.
Please take precautions in summer to ensure you avoid heatstroke. Ensure travelling arrangements avoid any stops where the hamsters will be left in a parked car -it is not sufficient to park the vehicle with a window open.
Ensure animals always have access to water or if that is not possible, to fruit or vegetables containing plenty of water, eg apple, melon, etc. Check the hamsters frequently en route. If you have to park choose a multi-storey car park if possible or a shaded area and ensure that someone is with the animals and checking them frequently while you are stationary. I have known animals die in a parked car with the owner in the car and the window open. You cannot be too careful when transporting animals in hot weather.
When housing animals in a shed or garage ensure that cages are not in direct sunlight. Ensure the building is well ventilated - ideally an open window/door at each end to facilitate airflow (cover openings with wire netting to prevent the entry of cats). These should be opened first thing in the morning before the heat starts to build. If possible paint the building white or a light colour, or grow plants up the walls to reflect the heat. If you have a choice in the positioning of the shed, choose a well shaded area.
I hope the above advice will help you in preventing heatstroke and give you points on first aid if any of your animals appear to be suffering from it.