Example sentences
gāi该shuì睡jué觉le了。
Time for bed.
tā他hái还zài在shuì睡jué觉。
He's still sleeping.
The story behind the character
睡
目 (eye) plus 垂 (to droop, also the sound). Eyelids drooping. That's sleep. Xu Shen wrote 睡, 坐寐也: "dozing while sitting."
觉
A shortened 學 on top, 见 (to see) below. From not seeing to seeing. That's waking up, realizing. Xu Shen wrote 觉, 寤也: "to awake."