Why was she able to sleep peacefully?

Knowing she had a plan, and knowing that Percy would return, allowed her to sleep peacefully. It was as if a wight had been lifed from her shoulders.

She felt almost happy now, and, drawing the curtains closely together again to shut out the piercing sun, she went to bed at last, laid her head upon the pillow, and, like a wearied child, soon fell into a peaceful and dreamless sleep.