Merino wool is da bomb - when outdoors I usually rotate between an Ibex mid weight top and a Smartwool slightly less heavy top. I like the feel of merino wool compared to synthetics or cotton. Both are comfy warm even when soaking wet, as long as I keep a wind resistant shell over them. I've slept in wet wool that way, once, but greatly prefer a fresher wool to change into if I can. Neither was too expensive, look for merino on sale in the late winter and early spring - I think the Ibex top was $35 from SteepandCheap, and worth every penny. Outdoor Research had a great sidewalk sale this spring where they had short sleeved Smartwool merino wool shirts for sale for $25. Pick your price point, pull the trigger...