The Collaborative International Dictionary of English v.0.48:

Daymare \Day"mare`\ (d[=a]"m[^a]r`), n. [Day + mare incubus.]
   (Med.)
   A kind of incubus which occurs during wakefulness, attended
   by the peculiar pressure on the chest which characterizes
   nightmare. --Dunglison.
   [1913 Webster]