It only takes one...well, three if you want to be thorough :-)
Half-asleep. Middle of the night. In the dark.
Your retina’s have two types of receptors, rods and cones.
Most of us are accustomed to how our vision works in daylight, when the cones make their contribution.
In the dark, most of your vision comes from the rods. So your night vision has lower spatial resolution, and is slower to follow movement. These characteristics become more pronounced with age.
The first muzzle flash will further exacerbate the situation.
If you are indoors (no starlight or moonlight) in the dark, unless you are going to be using NVD, I would suggest allowing a margin of error, i.e. more tries.
But God allows each of us to plan our own defense.
http://www.ncbi.nlm.nih.gov/books/NBK10850/