I too have seen them, but usually during the day and about the size of a cat or rabbit...I usually just associated it with my personal bunny and cat who had both died in recent years.
After doing some research, there is a general consensus that these shadows are from the other side...one person quoted Psalms 23...'though we may walk through the valleys of the shadows of death'.
They usually are just watchers, one person on another forum called them tally keepers, in that they watch us and keep track of our karma (kinda like santa claus). Other colors such as white a grey are common as well.
Numerous times it is suggested to cleanse your house when the visits get to common or creepy. Might I suggest purchasing some dried white sage and burning it like incense to ward off bad energy. Also clean house, literally; clear the cobwebs, dust the top shelves, get some windchimes, open the curtains, ring a bell. (these are all feng shui moves that increase energy movement). A stagnant house is a collector of sickness.
In your case, if this becomes a common occurrence:
keep a bell next to your bed, it, like a cough or windchimes will release bad energy or 'clear the air'.
Also just for fun, get a light within arms reach of your pillow. Mentally or literally tell it to go.
Bank the fire before going to bed.
Smudge yourself before bed. Or keep sage (or cedar, sweet grass, tobacco)under your pillow.
links for other herbs and purification techniques:
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