This is a question on the mind of many a reenactor, because so much reenacting goes on in the summer!

Our sources for medieval clothing in general are sparse, relative to more recent periods. Gale Owen-Crocker's Dress in Anglo-Saxon England, for instance, is based largely on the position of non-decomposable metal jewelry in grave finds, plus the scraps of textiles that have survived by clinging to them, and images from artwork, some of it rather crude, not made with the intent of recording details of dress and hairstyle. As a result, we often have to make conclusions based on fairly little or basic evidence. Owen-Crocker notes that just about all of the Anglo-Saxon fabrics that survive today are wool, which is in part a testament to the importance of the fabric to much of the European cultures at this time - but it's also because iron (used in most Anglo-Saxon jewelry) preserved wool better than linen, as did wet soils, and because linen was probably used for undergarments that weren't in contact with metal. She therefore has to speculate that linen could have been used for outer clothing in the summer, and that women in the earlier Anglo-Saxon period could have taken off the long tunic normally worn under the peplos overgown in hot weather. And it must be noted that wool's present-day associations with extreme warmth didn't exist in the past; it is possible to weave lightweight wool that breathes well in hot weather.

More detailed sources for everyday medieval dress tend to come later, in lavishly illustrated Books of Hours that contained pictures of people doing stereotypical activities for each month in the calendar. June, in the early fifteenth century Très Riches Heures, shows men and women working in the fields under the hot sun; the women are dressed in linen shifts (undergarments) and underdresses without longer sleeves pinned on, the latter tucked up to display their bare feet and legs, while two of the men are wearing just their upper-body undergarments and loincloths, also with bare feet and legs. This harvest scene from the Hours of the Duc d'Angoulême, 1475-1500, shows similar clothing: the woman is in her short-sleeved underdress, one man is in his shirt and hose without a tunic, and another is wearing a shirt and tunic but with rolled-down hose. As you mention, layering was key; but the thing is, you could wear the lightest layers all year round with heavier ones on top when they were needed. If your undergown was made with short sleeves, that would make it appropriate for the hottest days of the year, and longer ones could be pinned on at other times, for instance.

Something that reenactors often find is that our ideas of what makes people hot are affected by present-day practices for keeping cool during the summer. They say that not going from heat to air conditioned cold to heat again makes the heat easier to bear, and that long sleeves/skirts/pants often protect you from being too hot when you're in direct sunlight (pointing out that spectators in shorts and tank tops who ask, "aren't you hot?" are often feeling pretty toasty themselves). As of yet, I don't know that we can scientifically prove either of these, but it does seem likely that with the many changes to our culture and the way we experience our clothing and the weather, people from the middle ages did not feel exactly the same as we do now.

Follow up question to this i had read (i wanna say it was in wm. manchesters "a world lit only by fire") that peasants might go completely naked in warm weather. Is this true?