Chronic lack of sufficient sleep causes symptoms similar to intoxication and stress. Increased stress can bring on craving for comfort foods, which tend to have high caloric values. In addition, the fatigue caused by insufficient sleep often has people reaching for a caffeinated beverage such as colas, lattes, etc. which also have high caloric content. Finally, stress causes the release of cortisol in the body, leading to an increased propensity for gaining weight.

- quote (http://www.mayoclinic.com/health/stress/sr00001 ) -
The long-term activation of the stress-response system — and the subsequent overexposure to cortisol and other stress hormones — can disrupt almost all your body's processes. This puts you at increased risk of numerous health problems, including:

...
* Digestive problems
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* Obesity
...
- end quote -

Another relevant quote:

- quote (http://www.mayoclinic.com/health/food-and-mood/MY00716 ) -
Do you see someone who is sleep-deprived, gulping down caffeine and shoveling in fast food while on the run? Can you also picture the vicious circle at work here? Stress leads to sleeping less, which leads to reaching for caffeine and sugar for a fix, which is followed by a crash and need for another fix. Add to that skipping regular meals and exercise and maybe using alcohol to unwind. Alcohol and lack of exercise contribute to poor sleep. And so the cycle continues. We know that this way of eating doesn't make us feel good physically or mentally.
- end quote -

Like Opher mentioned, not sleeping enough causes your body to release cortisol and insulin which lead to overeating (typically of simple carbs like sugars and refined grains). Over time this leads to insulin resistance and Metabolic Syndrome.

In addition to this, your body does a lot of hormone balancing during sleep and as a result, not sleeping enough can affect your levels of Melatonin, Dopamine, Serotonin, etc. Most of these effect mood and your general health, so not getting enough sleep hugely detriments quality of life.

I recommend reading "Lights Out: Sleep, Sugar, and Survival" for an entire book on this subject.