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If you ever feel sleepy or lethargic during meditation practice, I’ve got some practical advice for you.
There are three reasons why you may experience sleepiness during meditation. I’ll address them one at a time, because each one is managed differently.
You’re not getting enough sleep
You may be sleepy because you’re not getting sufficient sleep. The human body requires a consistent 7 to 9 hours of sleep per night. If you’re not getting that, your physical, mental, and emotional health can suffer.
If you’re sleep deprived, sleep should take precedence over meditation. Take a nap instead of meditating, and take steps to get enough sleep every night. Then, come back to meditation.
If you suffer from insomnia check out the advice in this article by Chris Kresser. You might also like to try this deep relaxation to help you fall asleep:

Mindful15 Members can find longer deep relaxations on the Relaxation and Sleep page.

Peaceful Moment of the Week: Seagull with Clam, Cowichan Bay, Vancouver Island
 

Meditation is relaxing you too much
It’salso possible you’re falling asleep because the meditation itself is deeply relaxing. Meditation requires a balance of relaxation and alertness. If you’re too alert, you can’t let go of judgemental thoughts and reactions, but if you’re too relaxed, you will have difficulty being aware of the present moment.
Try some or all of these strategies to bring more alertness to your practice:

Begin the meditation with a few minutes of physical activity. Do a few yoga poses, take a short mindful walk, or shake your hands vigorously. 
Take an upright posture. Sit in a hard-bottomed chair (or on a cushion or bench made especially for meditation). Sit forward so that your back is not resting on the chair back or anything else. Sit a little straighter than normal, but don’t strain your back muscles.
Keep your eyes open. Focus your gaze on a wall in front of you, or on the floor a few feet in front of you. Let your gaze go soft and unfocused.
Make your in-breath a little more energetic than normal. Breathing in naturally brings a tiny bit of alertness to the body, while breathing out relaxes the body just a bit. If you slightly exaggerate the in-breath by giving it a bit more emphasis or energy, you boost alertness. Note, this doesn’t mean breathing faster or more deeply.

And, if these techniques don’t help, consider doing walking meditation instead of sitting meditation. You’ll find instructions in Episode 25. 
You’re sleepy because you’re resisting the meditation
The third cause of sleepiness during meditation is a sneaky one. It’s possible that your mind is resisting meditation and this resistance is unconsciously manifesting as sleepiness. 
There are many reasons for resistance. You might be worried that meditation will change you in some way or you might be anxious about the things you’ll learn about yourself when you practice. Sleepiness is one of the many ways resistance pops up. It’s your body’s way of protecting you from your fears about meditation.
Because you’re not conscious of the resistance underpinning the tiredness, it’s a challenging situation to deal with. Never fear, however, there is a fix.
The solution is to turn toward the sleepiness, thereby making the resistance conscious so you can explore it.