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The average adult needs 7-8 hours of sleep per night, and sleep is even more important when your immune system is compromised. Make sure that you're well-rested: don't stay up too late, and sleep in when possible. Sleep gives your body time to heal. Consider calling in sick at work--or going in late--to give yourself plenty of time to sleep in. You don't need to stay in bed all day unless you feel so inclined, but try at least to take it easy. It is easy to become dehydrated when you're sick, and dry sinuses will only worsen your cold symptoms. Drink plenty of water, tea, and soup to minimize irritation.  Avoid drinking alcohol and sugary drinks, as even low consumption can weaken your immune system. Wait to resume these habits until you're well and your immune system is ready for the beating. Consider using a humidifier in your bedroom to keep from breathing dry air at night. You can buy an electric humidifier at some department stores and drugstores. Your immune system is already compromised, so try to stay away from bacteria that might make your condition worse. Avoid hospitals, crowded areas, and other sick people; avoid any place where germs congregate. Wash your hands with antibacterial hand sanitizer several times throughout the day.  Consider carrying a small bottle of hand sanitizer about with you. Whenever you come into contact with germs or sick people, cleanse your hands. Avoid infecting others, especially children, the elderly, and anyone with a weak immune system. Cover your nose and mouth with your arm, a tissue, or a handkerchief when you sneeze or cough. Wash infected pillowcases, towels, clothing, and utensils to avoid reinfecting yourself once you're well. Sugar consumption weakens your immune system, and eating a lot of sugar-heavy foods may reduce your body's ability to recover from cold symptoms. There is some medical disagreement as to whether avoiding sugar during a cold can actually reduce the duration, but it is generally accepted that avoiding sugar in general is better for your immune system.  People tend to get sick at times when they're eating a lot of sugar: times of stress, and during the winter months. Stress itself weakens the immune system, so the combination can be dangerous. It is best to avoid excess sugar before these times, so that you don't compound the problem. Avoid candy, soda, and other sweets. Fruit juice is sugary, but it is also typically high in Vitamin C--just try to avoid juices that have a lot of sugar added to them. Many other animals are able to convert sugar into vitamin C, but humans cannot. Sugar competes with vitamin C in the body, so high sugar consumption often results in low vitamin C concentrations.
Get plenty of sleep. Stay hydrated. Avoid exposure to germs. Avoid sugar.