Laying down is normally a good thing if the rabbit has it's legs out. If a rabbits sits hunched it is not a good thing.
Please be careful if bringing outside bunnies indoors this time of year. Rabbits can not regulate their body temperatures very well and should not be subjected to extremes of temperature like low cold to homes with good central heating. It can cause many issues.
Rabbits grow thicker coats to deal with winter temperature so if you subject them to a heated room (especially in modern homes with UPVC windows where there is good insulation) they can overheat. A rabbit who is too warm may lay down and pant to try and cool down. Bunnies can deal better with being colder than hotter.
It then follows that if they are warm/hot and you put them back outside in the cold they can suffer too.
It can cause problems with their respiration.
During the day time it may not be quite so bad, but evenings - I'd not do it personally.
If you are going to bring a bunny in, please turn your heating off or right down ahead of his visit.
Helen