You'd want your highs in the mid 70's to get normal metabolism out of them, but nighttime temps can drop lower than you think... I currently have all my veileds outside, and our temps range from mid 50's to about 80 degrees (they've been out for a month). I'll keep veileds out up until around 90-95 as long as they have shade to get into, and they can be outside into the low 40's overnight, though I would never recommend it (I've had some overnight lows lower than expected with no ill effects).