I have a cotton sheet testimony! For years I have gone to bed and within 15 minutes my legs would start twitching, like classic restless leg syndrome.  My mom has always had "restless leg syndrome" so I didn't think to much about it.  Sometimes I could go to sleep and sometimes I would move to the coach and wonder why I could go right to sleep there.
After the TKM® training in Seattle, sometimes it takes several times to really hear something, I decided to buy some 100% cotton sheets which were jersey knit that I love.  The night before I put the cotton sheets on, I had a terrible time with my legs - as usual, but have not had that problem since I've gone to 100% cotton.  
A response: What on earth have you all been sleeping on if it wasn't cotton?  I'm 52 years old...and I've never had any sheets that were not made of cotton...
Response 1: The one big change we made with our son, age 23 with Down Syndrome, was recently to buy him an all wood futon with just a few screws in the corners and a cotton/foam mattress.  He slept through the night last night for the first time I can ever remember.  Before that he was on a metal futon frame, with an innerspring mattress.
Response 2: Most Sheets and bedding are either polyester or a polyester blend.  The average is a 50/50 blend.  This goes for pillows, blankets, comforters, etc.   Even when they say cotton. Read the details on the tag.  Also, anything that says "wrinkle free" is NEVER all cotton.  Note that blends have the same effect as all synthetics. It is BAD on your health - with all people!  It is also Biblical that we should not wear blends and science is now proving it is harmful to your health.  The evidence and practical application results are overwhelming.  If you want to learn more, attend a TKM® seminar and Dr. Glenn T. King will explain and demonstrate.