In my meditation practice, I have experienced periods of time where I think I would just be better off going back to bed. Thankfully, I have the awesome techniques from The Mind Illuminated by Upasika Culadasa. They have helped me move through drowsy times.
Dullness is one of the most difficult obstacles to manage when you are learning to meditate. It is something that will arise at different stages in your practice and can be a deterrent to your progress if not dealt with in a serious way. Catching dullness the moment you realize it is happening is key to preventing subtle dullness (a very pleasant state when you think you are in the zone but aren't) and a state called sinking (actually falling asleep or not being able to overcome heavy drowsiness) from setting in. Dullness can be sticky and difficult to overcome. Rest assured, there are tried and true antidotes to help you find that clarity you want in your meditation.
Lack of sleep, resistance on the part of your subconscious, and a reduction of the amount of information coming into your consciousness are all possible culprits. I will give you some suggestions to manage dullness that are very helpful.
See if you can detect the very first hint of dullness arising. Try to notice exactly where you feel it in the body. For me, it was around my eyes but it might be located differently for you. Note the qualities of how it feels.
At the first feeling of dullness, try one or more of the following antidotes:
Open your eyes
1. Open your eyes enough to let a bit of light in. In some meditation traditions, the eyes are always open. This was how I meditated for a while and it was helpful in reducing dullness. The difficulty for me was that I am trained as a visual designer and I become too interested in whatever is before my eyes. It took a while but I managed to be able to distance myself and not focus on what was in front of me. If you try this, the gaze should be downward a few feet or a comfortable distance in front of you. You do not want to feel any strain around the eyes as you do this. If you brighten up enough and would like to continue your meditation with the eyes closed, just close them and see if the antidote worked.
2. Take a big inhale and hold the breath for a few moments. As you exhale make a sound by creating resistance: hissing, pressing the lips together or fluttering the lips. It makes a sound so this might not be the best choice if you are meditating in a group. Repeat two or three times.
Tense all your muscles
3. Tense up all the muscles in your body thoroughly, hold for a few moments and then release. Repeat two or three times.
4. Stand up. Move slowly and mindfully always. Here are steps that I take so that I do not get lightheaded and I can keep my eyes closed. If you need to open the eyes, that is fine. Once you feel bright and you think you can remain so, sit back down in the same mindful way.
All of these antidotes will help bring you out of dullness at least for a short while. If it is only a temporary brightening, use the time to understand more fully about how you become dull so that the next time it happens you can head it off more quickly. The sooner you catch it, the more likely the antidotes will be lasting.
PRACTICE ON :)
Ahhhh, back to your seat with a vibrant attention and awareness!
My resources for this tip are from The Mind Illuminated by Upasika Culadasa and my own direct experience in using the TMI technique in my own practice.