My understanding of sleep paralysis (from my deep research listening to the Monster Talk podcast) is that it's an extension of what happens normally when you sleep: your muscles are temporarily disabled. That's a good thing, because it's what keeps you from strangling your wife when you dream about that. But sometimes your brain wakes up before your body does, instead of both happening concurrently. You haven't been able to move all night, but were unaware of it.
One factor that could be at play is you're not getting enough sleep and have to wake up before your brain is ready to. Most people don't get enough sleep.
Another factor could be caffeine. If I drink coffee late in the day, I'll have technicolor dreams, lucid dreams, and even compose melodies. Even though it's not keeping me awake, it's keeping some part of my brain active that should be chilling out. The next day I'll often wake up prematurely and feel like I've been up for hours already.