At those elevations, the temps can and do drop to the low 30's F, especially in June and in late August. I've seen snow above 6,000 feet in the Cascades in July.

Check out quilts from Enlightened Equipment, too. They have both down and synthetic. I assume, since you're a hammocker, you'll want a quilt rather than a sleeping bag.
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May your trails be crooked, winding, lonesome, dangerous, leading to the most amazing view--E. Abbey