眼が冴える (me ga saeru: to be wide awake)
eyeball + to be wide awake
Note that the first kanji isn’t 目 (me: eye) but rather 眼 (GAN, me: eyeball), the first kanji in this compound:
眼鏡 (megane: glasses) eyeball + lens
Wow, 眼 is just ripe for confusion with 眠 (nemu(ru): to sleep)!
As for 鏡, you may know it as kagami, “mirror.” The kanji 鏡 also has the yomi of KYŌ. Megane is a special reading (meaning, ateji reading) of 眼鏡, but you can also read it as gankyō, which matches the official yomi.