It seems that somehow people expect Spain to be hot, even in the Spring. While this may be true down in Seville, The
Camino Frances is in the north and passes through several high-elevation cities such as Leon (2,700+ feet), Astorga (2,850 feet) and several stages of the Camino remain at over 2,000 feet above sea level. Those nights are cold, and let me tell you, those refugios can be down right chilly, to say nothing of the monasteries. If you are an all-day walker, and you arrive late in the afternoon somewhere, chances are the beds will be full and you'll be on the floor, perhaps on a mattress provided by the refugio or perhaps on your air-mattress, and believe me it's cold. In July I woke up several times shivering in my flannel sleeping bag liner. I'll never travel again without at least a light-weight down sleeping bag. Nice and warm and cozy. Check the weight difference between a light-weight down sleeping bag (from Zpacks.com or Marmot, for example) and a sleeping bag liner. Often the difference in weight is just a couple hundred grams and boy, a nice warm night's sleep is worth its weight in gold compared to saving a couple hundred grams. I'll save weight elsewhere.
If you are thinking of bringing just a "sleeping sheet" or just a sleeping bag liner, I would invite you to try sleeping in it at home, on your mattress and also on the floor with a sleeping mat. Leave a window slightly open to simulate a drafty refugio and lower the heating so that the room is just slightly cool.
And by the way, I was just in Pamplona and it was snowing like crazy. All the mountains were white around the city.