A short time ago, I was sleeping in too late and going to bed too late, so to reset my body-clock, I got up early. I was tired. And I couldn't really meditate. I just kept falling asleep.
For some people, this happens often. If that is true about you, you're not getting enough sleep. And that's bad. Chronically undersleeping is bad for your health and puts stress hormones into your system. It's literally stressful to try to stay awake without enough sleep. Day after day it takes its toll. Death by all causes comes to the chronic undersleeper sooner. Your meditation is telling you something important: Get more sleep.
It should not be a struggle to stay awake when you meditate. If it is, make more time in your life for sleeping.
Another reason you would have difficulty staying awake is by trying to meditate during slump time, which is for most people around three in the afternoon. The human body is apparently trying to take a nap at that time. Your body goes into a slump and wants to sleep. Don't try meditating at that time.