Dullness and sleepiness are extremely common difficulties that arise in meditation practice. It may not seem like it but sometimes this can actually be a good sign! As the stream of thoughts, plans, memories and fantasies begins to quiet, the energy levels in the mind decrease; you can start to feel dull or sleepy without these usual sources of stimulation. The trick is to find energy in the meditation practice, and get used to having this vibrant alertness without thoughts.
These antidotes range from very light-touch techniques, to much more heavy-handed ones. It is best to start with more gentle antidotes, and work your way up to more intense ones until you find that the dullness has subsided.