Dracula, The Hunters' Journals: 13 August Mina Murry's Journal (Cont.)

Mina worries about Lucy and her sleepwalking. 

13 August.—Another quiet day, and to bed with the key on my wrist as before. Again I awoke in the night, and found Lucy sitting up in bed, still asleep, pointing to the window. I got up quietly, and pulling aside the blind, looked out. It was brilliant moonlight, and the soft effect of the light over the sea and sky—merged together in one great, silent mystery—was beautiful beyond words. Between me and the moonlight flitted a great bat, coming and going in great whirling circles. Once or twice it came quite close, but was, I suppose, frightened at seeing me, and flitted away across the harbour towards the abbey. When I came back from the window Lucy had lain down again, and was sleeping peacefully. She did not stir again all night.

Notes: Moon Phase: Waning Gibbous

Prior to the discovery of the Americas, the bat had not been associated with the Vampire. The Vampire bat of South America was quickly adopted as the animal form of the Vampire. 

I have noted already that Stoker was a big fan of America, so the idea that he would also use the bat seems like a foregone conclusion. 

The bat flies back to the Abbey, where of course Dracula is now staying. Mina "scarred off" the bat and likely was the also the cause of Lucy's good night sleep.