It’s in pet's health forum, so of course it's a valid question. Last time I was at a vet there was a girl with sick hamster, so if you can - take him to the vet, no one can know for sure what's going on without that.
The most common thing that causes diseases in hamsters are fungus and bacteria that he gets from his box and litter. If you notice dark patches on the bottom, where he sleeps, that's the fungus. But there is also one perfectly natural thing that can make hamsters lethargic or sleepy/ tired - they are one of the animals that hibernate and when they are in cold conditions they start to hibernate. Just bringing the hamster in the warmer room can make him wake up and he may shake or tremble before completely coming out of hibernation.
Wish you all the best,