To gain weight, eat food with extremely high caloric value. Try to increase your food intake without consuming too much fat and sugar. Regular meals, especially breakfast, are important if you find it difficult to gain weight. Taking your time with meals and making sure you are relaxed is also important to ensure that your digestive system functions properly. Have some fixed timings and avoid eating in between so as to increase the hunger for food. Avoid unhealthy, high-calorie foods. Most important is to avoid trans fats, which increase your risk for disease. Foods that are high in trans fats are: pastry products, cakes, cookies, processed meats, margarine, shortening, and packaged snack foods. Drink more water or juice. If you do not like to drink water, choose some healthy drinks like apple juice, orange juice, Take with less sugar etc. When you have to work late at night, these liquid materials will help you maintain your basic needs.