Tehirin: Pure Evil, Demons of Dreams
Based on their name and the context, the unique attribute of these creatures would seem to be their propensity to move about in daytime, a quality not generally associated with the demonic (Megillah 3a; Ber. 3b; Sanhedrin 65b).
[Attractive sleeping angel, New Orleans grave monument]
Never brought up in Talmud or early Midrash, they first get explicit mention Targum Shir ha-Shirim 4:6. 
Yet they receive the most prominent treatment in Zohar, where they are referenced multiple times (ironically, given their name derivation) as a creature that interferes with the night flight of the soul to heaven (Zohar I:83a). Perhaps there is another layer of irony, because the name closely resembles the Hebrew word for “pure” and “glittering,” yet these may be the very demons that trigger entrancing yet impure dreams in men (I:200a). Wittily, in his new Zohar translation Daniel Matt translates tehirin by the seductively charming alliterative “dazzling demons.”
(SEE: Did Satan Fall?: The Devil is in the Details
Lilith - semen demon or feminist icon?
Does the curse of Cain live on?
Spawns of Satan, Children of Cain )
 "Demon" is a catchall term for the many Hebrew terms for spirits such as sheidim, mazzikim, and lilin (djinns, imps, and night spirits). Yet the word is problematic, because these Hebrew terms do not carry same the infernal, satanic, essence of evil connotation of the English word "demon." While these spirits usually spell trouble for humans, they are as much like fairies and/or ghosts as they are like devils. Nevertheless, "fairy" has too mild a connotation in itself. Therefore I choose the word "demon" as a global term for all spirits in Jewish tradition that are not angelic.
 In the critical text, The Song of Songs in the Targumic Tradition, they appear as teiharei and is translated as "noontime ghosts."
 Pritzker Zohar, vol. 3, p. 162.
Zal g'mor - to learn more, read the Encyclopedia of Jewish Myth, Magic, and Mysticism: http://www.amazon.com/Encyclopedia-Jewish-Myth-Magic-Mysticism/dp/0738709050
Received this nice comment:
Dear Rabbi, I just discovered your blog, which for me is LivingWater. I look forward to your book; I'm wondering if it will supplant Joshua Trachtenberg's Jewish Magicand Superstition, which is my all-time favorite.I'll be joining the discussion!Be well.
Todah rabbah/Many thanks