Summarize the following:
Walk around a little if you can. Resist the urge to sit  or lay down. Also avoid leaning up against anything. If your eyes are heavy, you may doze off. As long as you are standing up and moving you should be able to avoid falling asleep. Cold water works best. The colder the better. Something to sort of shock you into waking up. You can easily excuse yourself to the bathroom to do this. The goal here is to make yourself uncomfortable. If your face is cold you will be less likely to fall asleep. If you have short, combed-over hair you could put cold water on your scalp too. This is a lot easier in winter or in cold offices. Take off that jacket or scarf. Roll up your long sleeves. Grab a glass of ice cold water. Take a few gulps, refill, and keep it at your desk. If you have a space heater at your desk, turn it off. You may even want to turn a fan on to keep you cooler. You may opt to start doing things more quickly. Stand and march in place while at your desk. If you can take a quick break, do jumping jacks or run in place. This will get your heart rate up. More blood will be moving to your brain and your lungs will start working harder to provide oxygen to the blood. As your heart rate rises you will be more awake and alert. Taking a shower will also influence your cardiovascular system by raising your blood pressure. If you are tired, a monotonous task will prompt your eyes to become very heavy. If you can switch tasks and do something different, try that instead. If it involves paying close attention to something or someone else that's even better. When our minds are engaged we become more awake and alert. If you can listen to something on the radio or on your phone do so. If you have a favorite radio station or upbeat playlist or music album listen to that and sing. Try listening to a book on tape. If you like keeping up with current events, news radio may also keep your mind engaged. If you are unable to shake your midday sleepiness you may not be getting enough rest in general. Consider establishing a regular sleep routine and avoiding snacks and beverages that can make you sleepy. Consult a physician if you have a regular sleep schedule and still feel tired during midday.

Summary:
Stay standing as much as possible. Splash water on your face. Lower your body temperature. Raise your heart rate. Engage your mind in an entertaining task. Eliminate the source of your tiredness.