Energy, sometimes translated as diligence, is the 3rd of the Buddha’s seven factors of enlightenment. In that context, it is said to arise as a result of practicing mindfulness and it’s related factor, investigation of phenomena.
It’s interesting that the Pali word for this factor can be translated as either energy or diligence. The meaning is that when we are diligent, we create energy, and likewise, when we have energy, we act diligently and with care.
This has implications for meditation practice. Sometimes in our meditation, we feel drowsy, and we may be tempted to just go to sleep. And sometimes that is exactly what we need. But more often, we are simply drowsy and not really tired. In that case the best thing to do is to expend energy so one becomes more wakeful. Check your posture, and sit up a little straighter. Hold your hands up off you lap with your thumbs touching and your fingers slightly overlapping. Doing things like this can wake us up.
Many times we feel tired in our lives. Perhaps we come home from work exhausted, and all we want to do is sit in front of the TV. But if you can get yourself to engage in some walking or other exercise of some kind, you will find that you aren’t really tired, just drowsy. You will feel refreshed and alive.
The principle is: to get more energy, expend more energy.