Hypoglycemia (low blood sugar) can cause many physical and psychological symptoms including glazed eyes.
It is also said that glassy eye is caused by not blinking often enough, causing your eyes to become "dry" and lifeless.
Taking drugs which affect the central nervous system, and even some medications, can give your eyes a glassy look as you tend to blink less frequently if the drug is actually depressing the nervous system. And most people know that marijuana often results in glassy and red eyes...
Glassy eyes is one of the criteria for determining if a person is drunk while driving. Alcohol depresses the nervous system, and affects the eyes functions, such as size control and blinking.
Treatment for Glassy Eyes
Placing a couple of Visine or Rhoto eye-drops in your eyes can take away some of the glazed look fairly quickly.
You can also try using of natural tears 4-6+ times/day to assist with your natural tear production.
If you are concerned about having glassy eyes your doctor can diagnose the cause and take measures to address it. For example, if your doctor diagnoses you have hypoglycemia, then treatment would involve raising your blood sugars levels, and finding what's the cause of the condition itself.