Humans are not designed to be nocturnal; therefore the human eye is not as well suited to nighttime viewing as a night animal, say a cat’s. The reason for this inability to see as well at night as a cat is that the pupil of the human eye does not open nearly as wide as a cat's. The pupil is what controls the amount of light allowed into the eye. Naturally, for seeing in the dark, the ability to let in as much light possible is important. If you look at a cat in a relatively dark setting, you will see that the iris, the colored part, of the cat’s eye is almost completely blocked out by the black pupil. This is because the pupil is almost completely open, which lets in the most light possible, which translates to excellent night vision. In bright sunlight, a cat's pupil appears as a tiny black slit in the middle of the colored iris. Unfortunately, for naked eye astronomers, the human pupil can not open up to the amount that a cat’s can. The good news is that the human pupil can open up to a certain degree, the bad news is that this process takes about ten to fifteen minutes. So, before stargazing, give your eyes at least ten minutes to adjust to the dark. Once you get used to looking at the stars, you will notice a difference. Many more stars will be visible after ten minutes than after just after stepping out of your probably well-lit house, human night vision at work.
Once optimum night vision has been achieved over the course of about ten minutes, a split second can ruin it. Since the human eye is more adept at picking up light than seeing in the dark, the pupil quickly dilates, or closes at the sight of a bright light. Once dilated by a bright light, it will take time to re-achieve night vision, with time being directly dependent on the intensity and color of the light, which is also an important consideration.
The spectrum of visible light runs from red, orange, yellow, greed, blue, indigo, and violet (thing 'Roy G. Biv'). Colors on the red end of the spectrum have longer wavelengths and less powerful frequencies than the short wavelength light on the violet end of the spectrum. Since the power of the wavelength can vary with color, it should come as no surprise that reddish colors with longer wavelengths at lower frequencies are less damaging to night vision than other colors. This lesson on the color spectrum explains why you will only see red shaded flashlights at any gathering of observational astronomers.
Getting back to night vision, it is a good idea to go behind your house if you live in a suburban setting to avoid all of the car headlights going up and down the street, with each pass re-ruining your night vision. In addition to escaping from headlights, escaping street and house lights can do wonders for the amount of stars you will be able to see.