It's very important to have a proper sight picture & proper sight alignment before firing your handgun at your target. You should also train with all your firearms in both light and dark environments so you don't panic under stress in a situation you aren't ready for, such as shooting in the dark and suddenly being unable to find your sights.
This post applies to common 3-dot sights. There are a lot of other sight options out there that require different sight pictures, such as Heinie Straight Eight Sights, which have 2 dots, or XS Sights, which have a front dot and a V notch in the rear.
There are also different ways to position your sights onto your target (depending on what you prefer or whether or not your pistol sights can be adjusted), but this is the sight picture style I train with: by centering the top of my sights into the center of the target.
For target shooting, I focus my eyes on the front sight - This leaves my front sight clear and in-focus, while my rear sight becomes slightly blurred and out-of-focus. For fast, defensive shooting, I instead focus my eyes on the target - This leaves my target in-focus, while my sights are slightly out-of-focus (but the 3 dots are lined up on target).
While slow target shooting for accuracy, sometimes I will close one eye. For quick, defensive shooting, I always practice shooting with both eyes open.
Some people focus on their sights and target differently, but this is what works for me.
Line up the front sight post in the middle of the notch in the rear sight. Keep the top of the front sight post level with the top of the rear sight, while keeping an equal gap of daylight on each side of the front sight post.
IN DARKNESS, with NIGHT SIGHTS:
While shooting in the dark with night sights, you will not be able to see lights gaps on the sides of your front sight post very well, if at all. In this case, line up the glowing dots on your night sights so they are all level across the target before firing.
IN DARKNESS, with PISTOL LIGHT:
While shooting with a light attached to your pistol, you won't be able to see the dots on your sights well, so you have to get the same sight picture without the dots. Line up the front sight post in the middle of the notch in the rear sight (just like you would in daylight). Keep your sights level across the top, while keeping an equal gap of light on each side of the front sight post.
Night sights are extremely important for low light shooting and are a great investment. You should also have a light attached to your pistol (if possible) or have a hand held light for low light situations so you can identify your target.
Regardless of whether or not you have night sights or a pistol light, you should train and be able to shoot in low-light situations in case you ever have to.
Note - There are many options for handgun sights. Some will have a larger gap (on each side of the front sight when lined up with the rear sight), some will have a smaller gap, and some will have hardly any at all. Not all 3-dot sight alignments will look exactly like these photos.
All 3 of these images were taken out of my videos on the following videos on the SIG 1911 Extreme & the SIG 1911 Spartan: