In general, you should strive to not have any drugs – whether it be caffeine, nicotine or other drugs – in your system before practicing. You’re goal is to have your body and mind be in a natural state so that you can work with that part of your consciousness and spirit.
However, the most important rule of Qigong applies here:
Be aware of your own body, and listen to what it tells you.
Every person is different, and every day is different. If you feel you need caffeine just to concentrate and feel more like your natural self, then it might be appropriate.
While caffeine is a good mental stimulant, it also raises your blood pressure. Qigong breathing and moving will lower your blood pressure, but you might be sending your body on a small rollercoaster of blood pressure. This is why it’s important to be aware of how your body feels.
Once you have practiced Qigong for awhile, you’ll find that you don’t actually need the caffeine in coffee to be awake and present during Qigong practice. Once you begin to relax and breathe, you’ll naturally feel more calm and alert (as alert as you need to be).
And, if you are so tired that you need coffee to physically stay awake and practice, try this ancient trick instead: Go to sleep!