A DUVET (GB)
A CONTINENTAL QUILT (UK)
I myself sleep under a duvet although I don't mind using something lighter when nights are really hot. If you feel cold at night, I guess you should invest in a good eiderdown (kołdra puchowa). If you're allergic to saprophytes, you could invest in an acrylic duvet (kołdra wypełniona anilaną). And finally, if you suffer from arthritis... they say... the best one is a wool duvet (kołdra wełniana).
What do you cover yourself with?