The two main ones are (1) cotton doesn't make you sweat, as it's a 'breathable' cloth - and (2) it doesn't create static if you're sleeping on nylon sheets !
the price of slaves increased dramatically through mid century. Slaves in their 20's who were regarded as a good or even fair hand were valued at $700 or $800.
Cotton is used for sleepwear because it is comfortable. When you are sleeping you want material that won't irritate sensitive skin, and won't cause allergies. Cotton is used so often in sleepwear...
I am doing an essay on this right now so I would be glad to answer your question. The positive effects of the "cotton gin" was that it made producing cotton so much easier. Before Eli Whitney...