Cover the window with a blanket. I know it sounds silly but I've liver in some super col flats and it helps. Get a fairly light one, mine was standard ikea fleece one, then pin/tape it to the top of the window, as close to the glass as you can. During the day keep it up for the vent an general niceness and at night it won't be so tight that you totally cover the vent. When it's down you might need to attach it at the bottom and middle to not have it too loose
Nothing wrong with sleeping in more too. I have quite thick fleece PJ bottoms from Matalan that are magic, with a vest tucked in then a loose hoody over the top and fluffy socks so it's still all nice feeling to sleep in