Cotton and Tencel are both naturally breathable materials, that will work to regulate your temperature as you sleep. However, Tencel is known for having a soft, silky texture that makes it cooler to the touch. Its generally recommended for hot sleepers thanks to eucalyptus naturally being moisture-wicking and antibacterial. Cotton can be cool but only if it has a percale finish. This is still thicker than Tencel bedding so would generally be better for all-season use. I've personally found Tencel to be just as breathable as cotton, with additional cooling features that allow you to feel and sleep fresher.