what should i do if i feel sleepy while i meditate?

Feeling tired during a meditation session is quite common.
One of meditations greatest benefits is its ability to calm us down and put us in a state of tranquility. That fact that you feel tired while meditating may just mean that it's working!

At what time do you meditate?
If you meditate close to bedtime, or right before, as many people do, then it makes sense that you will feel tired. When we go to sleep at the same time, or at a similar hour each night, we are basically training our bodies to understand at what time it should wind down and prepare to fall asleep.

If you choose to meditate around this time, you may just be giving your body extra signals that it needs to get ready to fall asleep. This is because in addition to it already being trained to know that it should be falling asleep soon, you are also giving it all the calming and relaxing signals meditation gives, which may amplify the bodies response and make you even more tired and sleepy.
If possible, try to meditate a little bit sooner so that you are not meditating right before bedtime.

If you meditate right in the morning, shortly after you wake up, and also feel tired, it may just mean that you have not fully woken up yet in order to meditate.
Meditate is a calming exercise, if you wake up in the morning still feeling tired from the night's sleep, and then you engage in a calming and relaxing exercise like meditation, your body may just continue to be sluggish and feel tired, causing you to feel even more tired than you may be after waking up.
If this is the case, try to meditate a little bit later if possible, or, if you are one to drink tea or coffee in the morning (or any other morning drink to wake you up), try meditating after drinking it.

Do you often get tired while sitting down?
There are certain health conditions that cause people to fall asleep and get tired pretty instantly. If you experience tiredness throughout the day while sitting, it may be worth seeing a health expert about this and explaining the situation.