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The main reason why you get sleepy after lunch is because food that you eat for lunch diverts your blood away from your brain to help with the digestion process. Your body also releases a bit of melatonin after lunch related to a drop in core temperature that occurs around 2 to 3pm. Melatonin is a hormone that helps you fall asleep at night. A post lunch slump may be worse if you haven't had enough sleep the night before. Adults need about 7 to 8 hours of sleep per night to function at their best, so try to go to bed in time to get enough sleep each night. If you are suffering from insomnia, talk to your doctor to determine the cause. While a post-lunch slump is normal, inadequate or poor nutrition may make your afternoon drowsiness worse. To determine how to avoid feeling sleepy after lunch, consider the following questions:  Do I eat breakfast every day? Have I been eating a lot of processed and high sugar foods? Have I been eating poorly the night before? Have I been consuming too much caffeine and alcohol? Is my breakfast providing me with nutritional energy? (more than just coffee) Have I been physically active consistently? Do I have a right life-work balance? Have I been eating healthy lunches? If your answer to any of these questions is no, then you should evaluate your lifestyle habits to help make your post-lunch slump less severe. Write down when you feel drowsy, what you ate, whether you had exercised or not, how well you slept the night before, and any other factors that might be involved. Do this over a week, and at the end of the week, analyze the data you have recorded. Look for patterns so that you can learn to avoid any habits that cause drowsiness problems for you.
Be aware that feeling sleepy after lunch is related to digestion. Consider how much sleep you have been getting. Ask yourself if your eating habits may be contributing to your afternoon drowsiness. Track the habits that make you sleepy in a food diary.