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Mine have something to do with muscle tension. I find that I only get a good sleep with one of those memory foam pillows, specifically the ones shaped like this: https://www.amazon.ca/Obusforme-Contoured-Memory-Foam-Pillow/dp/B000FJQYZ6/ref=sr_1_15?ie=UTF8&qid=1541703571&sr=8-15&keywords=memory+foam+pillows
without it, I'm much more likely to wake up with neck problems and/or migraine.
Also, I have to keep my neck and shoulders almost uncomfortably warm; the tiniest chill will trigger muscle tension, and /or migraines, so I sleep with a thin hoody because blankets fall off.