Thanks all for your replies. I am familiar with the recommendation of hiking during the early morning / early evening hours through the desert, and that would definitely work well. I was mostly intrigued by the idea of sleeping only during the day, since when I look at my gear list it appears a huge amount of the weight is devoted to staying warm while sleeping. I just wanted to propose the idea since I haven't run into it in a lot of months of PCT planning.
If I drop all of the extra gear devoted to sleeping warmth (warm sleeping bag, thicker pad, microfleece pants, etc.) I find that its really easy to break the sub-5lb barrier using gear I already own. It's more "elegant" than anything else I suppose, but an attractive idea nonetheless. The main problem I do see is being able to sleep well during intense heat, but maybe if I were to find one of the cooler spots you mentioned, Piper.
I have done some night hiking before: I did a good part of the Presidential Traverse in the White Mountains at night, which was OK but I wouldn't do it normally. I do really like tromping around nearby trails in the woods here in Michigan at night, but albeit not usually by myself. I guess it would be a lot more fun if I had a partner. To me, it makes the wilderness feel even more "wildernessy". Sleeping from, say 10am-6pm would allow me to hang out with other people in the mornings and evenings, if I were to do it that way.
The points about scenery, and sleeping while its hot are very well taken. I just wanted to float the idea, and maybe I'll try it sometime here at home before I do it anywhere else.
Also: I have a question about the emergency blanket. Would it be possible to tape/bond the mylar blanket to a layer of ripstop nylon/M90 type material to make a nice comfy blanket against the skin, or would the presence of the nylon ruin the functioning of the mylar material? I'd expect it wouldn't, I just don't really know how the things work.