People often slept nude. In cold weather, they might sleep in their undergarments. For men, this might be a loose-fitting arrangement not unlike boxer shorts or pajama bottoms, coupled with a long (by our standards) undershirt. Women typically had an even longer undershirt. One might wear these underclothes for a solid week before changing them.

One thing to remember is that materials were not cheap, and making clothes by hand was a labor-intensive process. For most people, it would not have been practical to own a shirt for the sole purpose of sleeping in it.

Singman's Daily Life in Medieval Europe is a handy reference for stuff like this, and it comes with a substantial bibliography. Most of his source material seems to come from the 14th century, which I personally feel qualifies as "early Ren." In any case, it's not like a switch was flipped and agricultural workers suddenly became wealthy. I feel like I'm on fairly solid ground in saying that, on a fundamental level, the standard of living enjoyed by the average Northern European wouldn't have changed much between c.1350 and 1500.