I tried a few things last year, but room temperature is the main thing
It means having your heating on more, on low maybe. I had a heater on low during the night in winter and used to come down and find Charlie lazing in his sputnik near the heater - he liked being warm!
In Autumn when it's just the odd chilly evening, something like a heat pad would help but I reckon in the next couple of weeks or so it will be a case of needing a heater. They are affected by cold and are awake all night when most people have the heating off.
Someone recommende one of these last year and I got one. I propped it up against the end of the cage next to Charlie's hamster house but it was only really any good for this time of year.
https://www.amazon.co.uk/gp/product/...?ie=UTF8&psc=1