We think of light in many ways: sheer illumination, texture, drama, and artistry. We first learn to light the subject, and later the environment. Then we return to lighting the subject with greater nuance. Today I'd like to think of light as time travel. By that I mean light can place a moment in any time of day (or night).
Our minds are so accustomed to interpreting light that it's completely unconscious now. We see long, sideways shadows and immediately understand the time to be early morning or late afternoon. Short shadows indicate noon time.
Even though we can't see these women's shadows on the ground, there's no question that the sun is either rising or setting in this image. Which do you think it is? If you guessed sunset, you'd be right. But how did you know?
We can re-create that time of day using flashes or continuous lights. The well-advised adage is "when the sun is high, place your subject in shade." When that shade is too dark (say, beneath a large building) the resulting image can lack contrast. Adding a flash can give your images vibrancy, depth, and also a place in time.
The image below looks natural, but there's a Nikon SB800 in a 24" soft box camera right to give it a little more oomph. Notice the soft shadow under her chin and camera left. For this portrait, I wasn't concerned with time (though from the bright spot on the wall immediately behind her we might guess that it's late afternoon). I was focused on depth and texture. Our minds, however, are seeing time in the picture. The light is imitating mid day under a light shadow; only the rectangle of sunlight on the wall behind her head (camera right) says otherwise.
Time of day isn't just shadows, though. Long shadows without the corresponding change in color temperature will look fake. Mismatched light temperatures are an annoyingly common mistake when photographers combine flash portraits with sunsets. The color temperature on the model is 5200K, while the sunset light is closer to 2000K (the solution: gel your flash-- you can correct this problem in post but it's a pain).
Once you've created your shadows, adjust your color temperature. Flash is a daylight temperature of 5200K. Warm it up to 3500K with an orange gel and see how much more natural your sunset portraits are. The difference in light temperature between sunset (roughly 2000K) and your gelled flash (3500K) will make your subject stand out without appearing unnatural.
Now that you're attuned to time-cues in images, browse Flickr, Instagram, or your favorite photographer's website and practice identifying the elements that create time of day, and the mood it evokes. Check out mine, www.hurricaneimagesinc.com, and let me know what you find.