I just got a
Castelli Summer Skullcap. I've worn it just a bit so far, and I like it. Until now, I've been wearing cycling caps.
Everyone's different, but I think I might find I prefer not to have cotton because it holds the sweat there. Wool and synthetic let it wick away. I've been wearing a lightweight wool cycling cap (from Walz), expecting it to make me hot, and it doesn't.