Dull winter blues

Hi. Our little buns are full time house bunnies, and I was wondering that since daylight at this time of year is so minimal is it possible that this can have an effect on the little ones? The room that they live in when we're out during the day can get quite dull sometimes, and i've noticed a little bit of a change in the buns temperament. The bunnies health seams fine, they're both eating, pooing, drinking etc.....but they're just a little bit lazy at the moment, lying fully down, head low.

Does anyone know if there is anything I can give them, such as vitamin supplements during the winter dull months to put a little more spring in their steps?

thanks