There are two different approaches to meditating when one’s energy starts to drop (Sloth). One is to turn the attention to the experience of low energy itself and make that the object of meditation. The other is to raise the energy.
Here are some ways to raise the energy from the most subtle to the most gross intervention:
Naming or noting the experience in your mind.
If meditating with the breath, pay more attention to the in-breath, as the in-breath is energizing. Pay less or no attention to the out-breath, as the out-breath is tranquilizing.
If you are meditating with the breath, shift your attention to the body and move it between touch points. Hands touching, sit bones touching, and feet or legs touching, for example.
Raise your chin up.
Straighten your posture.
Take three or more breaths that are slightly longer and deeper than normal.
Open the eyes.
Raise your arms over your head, like a referee signaling a touchdown during a football game.
Pull down on the lobes of your ears.
Stand up. Be sure that the knees are not locked.
Do walking meditation.