If you fall asleep, it is usually the sign that you body needs sleep.
You actually need a nap.
So, feel free to give it to yourself.
It can as well be the sign that the way you approach your practice is not dynamic or exciting enough.
Meditation is not a lethargic activity where you enter into some form of passive apathy state.
If this tends to happen, shift your practices to short dynamic sessions.
Focus on 5 minute practices rather than hour long meditations.
Energize your body with some dynamic breathing techniques or dynamic moves before you meditate.
You can as well practice outdoor which is very uplifting.
If you are indoor, make sure the room is not overheated.
Open the windows and get some fresh air.
The simplest thing to do though is to get more sleep.
If you are hungry, listen to your body and give it some food.
If you are sleepy, take a moment to relax and let go.
Trying to suppress or fight with these basic needs can give you a short term “victory” (with use of will power) but on the long term, listening to your body and natural needs works better.
No need to suppress, no need to block or lock yourself in some form of rigid discipline.
Follow the natural currents and rhythms of your nature.
It works better.
You can work hand in hand with your instinctual nature and body needs rather than against them.
The moment you do, you will discover a new sense of easiness in your practices and approaches.