I agree with h3rmit, definitely sit with your back straight as the tonus keeps you vigilant and remember to try to stay alert, it takes practice.
This is a common occurrence because the high frequencies tend to knock one out so to speak. It's especially common in transmission meditation. Yawning is always present in tr.m. groups and quite a few individuals doze off within seconds of tuning in! It's sometimes even advised to practice transmission meditation standing up! Still, I would remain seated and try to stay awake for as long as possible and gradually stay awake for a bit longer etc. Basically, as you keep practicing you get accustomed to the energy flow and develop a better light body and hence are able to stay awake until you're ready to conduct even higher frequencies- this is when they tend to knock you out same as the first ones. Be prepared that once you get used to the energies they might have the opposite effect. With before mentioned transmission meditation some practitioners have trouble falling asleep for hours afterwards once they've gotten used to the flow.
It just takes some time getting used to, good luck and let us know how it goes