Originally Posted by khuon
It can be worn under a helmet although you might get a little hot. It works wonders when it's colder though... certainly better than stuffing newspapers in the vent holes.
Well, "colder" isn't much of an issue here - since I live in Hawaii.
It does get cool enough at night (I ride at 3:00 AM) to justify arm warmers and a very light jacket, but I've never felt my head was too cool. Even if it was good for that part of the year, it would be a small part.