There can be many reasons to feel sleepy while meditating so it's hard to pin point it exactly.

Does anything else give you a similar sensation of sleepiness?

Sometimes it's the mind wanting to phase out, sometimes we forget to breath, sometimes it's because we have a full stomach, sometimes we are dehydrated, sometimes it's a phase that passes quickly, sometimes not, sometimes we just need to sleep.

Are you able to catch the sleepiness and focus on that? exploring it consciously?

In some cases simply a more movement based practice helps develop some mindfulness comfort in an active way before we can explore different types.

So in short it's normal and there are many reasons why.
Please let me know if you have questions along the way.

These are some playful active meditation practices you might like.