I'd like brass. Whenever you're too hot, you can just cool yourself off. Later, when you're too cool, you can just warm yourself up. My only issue is that the heat to cold ratio would be very disproportioned, as where I live I would need to be warmer more often than needing to be cooler.
Bronze would be nice too (wakefulness). I'm tired all day, so if I stored up all my tired energy all day so I can be alert, then at night I can BAM, get a good night's rest. At least I hope that's how it would work.