Recently while surfing the net I came across the information on coffee which amazed me. There is a chemical called adenosine which makes us sleepy. Coffee helps us to clear this chemical of our system for a specific time. So when we have coffee we feel alert and refreshed. we can stay alert for longer period of time if occasionally we drink coffee. It is also suggested that if we take a siesta after drinking coffee we instantly we feel alert for longer duration of time. It is named as caffeine Nap where drivers were tested for their long and stressful hours of work to determine which method is best to stay up alert for long. 

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