from a talk on the Factors of Enlightenment:
Now these different factors of enlightenment are real important, while you’re sitting in meditation. The Satipaṭṭhāpana Sutta makes quite a big deal of the seven factors of enlightenment as part of the dhammānupassanā. And how do you use these things?
When your mind is dull, and there’s sleepiness in your mind, then you have to call up your mindfulness factor, you have to be able to investigate how that dullness arises, and the more interest you have, the more energy you have. When you finally have that balance of mind where there’s enough energy and the sleepiness and dullness go away, joy will arise, all by itself.
If you start to put a little bit too much energy into your meditation practice, you can cause your mind to get very restless, and with that restless feeling, it’s an unpleasant feeling.
There’s some dislike, and feeling of your mind is really active, very thoughty.
The way you overcome the restless feeling is by focusing your mind on tranquility, and composure, having calmness in your mind, and as you do that, you will start to get more and more equanimity arising.
Now as you go higher in your meditation, as you go deeper into your meditation, you will start to become more and more sensitive to the amounts of energy that you need to keep a good balance, especially if you get up into the arūpa jhānas, it becomes very, very fine.
If you don’t have quite enough energy, your mind will get dull. If you have just a little bit too much energy, your mind gets restless. So you get to learn more and more finely tuned consciousness, and the way you develop that is through the enlightenment factors.
The more alert your mind becomes, the more you really notice the kinds of energy you need to keep to sustain the feeling of equanimity, of peacefulness, of calm.