Drowsiness can have different causes depending on the type of person. One is the quality of that person’s sleep in the night. Sometimes you might have slept for long hours but if the sleep is not deep and relaxing, during the day you can feel sleepy as a result.
Another reason for feeling drowsy in meditation is the quantity of food that you have eaten. Sometimes if you have eaten too much or too little it is also possible that you might feel drowsy.
Another reason which I have discovered while working with meditators is that sometimes they don’t want to see what is arising in meditation. So sometimes sleepiness can be used as an excuse not to look at unpleasant things that are arising. It’s an interesting reason. And sometimes when we feel relaxed while we are meditating, then also sleepiness comes.
So whatever the reasons are, one thing that can be attempted is to change the posture. This is one of the recommendations of the Buddha also. So if you are sitting, you can do some walking meditation, you can do some very quick walking meditation, or you can try to walk backwards. The idea is just to induce more energy using walking meditation.
Another thing that can be attempted, and this is where awareness is important, is to sharpen your awareness so that when signs of sleepiness are coming, you’ll be able to catch it and then do something about it, either open your eyes or stand up or start walking.
The third thing is that sometimes when your effort is too weak, you can feel sleepy. So then you can make more effort to sustain awareness, you can try a little harder.