1 Intellectual work makes calories more desirable
According to one study in the journal Psychosomatic Medicine, students who did a text summation consumed 203 more calories than students who relaxed. Even though the students didn't burn more calories they just felt like eating more.
2 Fluctuation in glucose and insulin
According to the same study, the process of thinking causes fluctuation in the levels of both glucose and insulin. This could result in sending the signal of hunger to the brain.
3 Concentration requires glucose
When the brain concentrates it requires more Glucose as it quickly consumes the Glucose in the blood stream. This leads to the feelings of hunger even if the body didn't burn much calories.
4 Lack of sleep
Many students sleep less during exams, and as a result their hormonal levels might get disturbed. This kind of disturbance can lead to overeating and obesity. (See How to Have a Good Night Sleep?)
5 Stress leads to hunger
Under the effect of stress caused by exams and studying a student might feel like eating in order to feel better. This kind of emotional eating can regulate the mood. (See Why Do Exams Cause Stress?)
6 Thinking only burns 3 more calories
Studies found that thinking sessions burn only 3 more calories than relaxing sessions. It turns out that the fact that studying makes people hungry has nothing to do with actual burning of calories, and as a result some people get obese on exam days.
7 Availability of food
The fact that most students study at home makes food more available to them, and as a result a person can easily eat more than he needs.
Boredom can lead to overeating, as fatty foods stimulate the body to produce certain chemicals that can help the person feel better.
Some students eat a lot during exams because food breaks allow them to procrastinate or stop studying without feeling bad or guilty.
10 Distractions lead to hunger
According to one study, visual distractions such as the TV and probably studying material, can force a person to eat more as his brain might fail to realize the amount of food he actually ate.