If you don't care about pockets, and wind resistence, a "technical" t shirt (i.e. synthetic fabric that wicks moisture) works well.
Mountain biking with a camel back (obviating the need for pockets) I'll wear a t shirt.
__________________
You could fall off a cliff and die.
You could get lost and die.
You could hit a tree and die.
OR YOU COULD STAY HOME AND FALL OFF THE COUCH AND DIE.